Ministers have warned of 7,000 truck-long queues in Kent after the Brexit transition period ends as a worst-case scenario if hauliers fail to prepare for changes to customs rules.
The Chancellor of the Duchy of Lancaster Michael Gove, responsible for no-deal planning, has written to logistics groups with the Government's "reasonable worst-case scenario" planning, which warns of possible two-day delays for cargo travelling to France in January.
Mr Gove is due to outline the scenario work, which the Cabinet Office stressed is not a forecast, in the Commons on Wednesday.
It comes as the European Union's chief negotiator Michel Barnier prepares to travel to London for further informal talks with his counterpart Lord Frost as efforts continue to strike a post-Brexit trade deal.
But in the document sent to logistics associations, which has been seen by PA news agency, Mr Gove warned that changes were coming with or without a deal.
Pictures of the week: September 20 - 26
Pictures of the week: September 20 - 26
A piece of street art of two girls on Hawley Street.
A huge variety of Street Art of all kinds can be found almost around every corner in one of London's artistic and cultural hotbeds of Camden Town. (Photo by Keith Mayhew / SOPA Images/Sipa USA)
Captain Sir Tom Moore in Wootton, Marston Moretaine, Bedford with veteran fundraiser Lloyd Scott, who will attempt to climb the Three Peaks whilst wearing a deep sea diving suit. (Photo by Jacob King/PA Images via Getty Images)
Manchester City's Adrián Bernabé reacts as he is carried from the pitch during the English League Cup third round soccer match between Manchester City and Bournemouth at the City of Manchester Stadium in Manchester, England, Thursday, Sept. 24, 2020.(Mike Egerton/Pool via AP)
A man dressed as Santa Claus is seen seated in a picnic chair with a poster in his hands around Leicester Square in London to promote the new channel Sony Movies Christmas which launches on Thursday the 24th of September and will run for the next three months. (Photo by Brett Cove / SOPA Images/Sipa USA)
A shopper wears a mask as she exits a shop in Windsor, England, Wednesday, Sept. 23, 2020. British Prime Minister Boris Johnson appealed Tuesday for resolve and a "spirit of togetherness" through the winter as he unveiled new restrictions on everyday life to suppress a dramatic spike in coronavirus cases. (AP Photo/Kirsty Wigglesworth)
A visitor looks at George Morton-Clark's artwork during the exhibition.
Opera Gallery, in London's New Bond Street, presents a show of new works by British artist George Morton-Clark, entitled "This is as far as I started". Following the devastating blast in Beirut, Opera Gallery London will be showing their support by donating part of the proceeds from the new exhibition to the Paediatric Ward at St. Georges Hospital in Beirut. (Photo by Keith Mayhew / SOPA Images/Sipa USA)
People wearing face masks wait to enter a store at Regent Street.
Members of the public wearing protective face masks as they shop in Central London as further lockdown measures are proposed for the capital later this week. The UK government is preparing for a possible second nationwide lockdown to fight the spread of COVID-19. (Photo by Keith Mayhew / SOPA Images/Sipa USA)
A person looks around the Mayflower 400: Legend and Legacy exhibition, during a press preview for the UK's largest ever commemorative exhibition on the Mayflower, with works on loan from both Smithsonian and Peabody institutes in America, at The Box museum in Plymouth, Devon.
A staff member of a toy store wears a face mask as she invites customers to enter the store, in London, Tuesday, Sept. 22, 2020. British Prime Minister Boris Johnson has slammed the brakes on the country's return to offices, saying people should work from home if possible to help slow the spread of the coronavirus. Johnson on Tuesday announced a package of new restrictions, including a requirement for pubs, restaurants and other hospitality venues in England to close between 10 p.m. and 5 a.m. (AP Photo/Frank Augstein)
EMBARGOED TO 0001 WEDNESDAY SEPTEMBER 23 EDITORIAL USE ONLY Quality control manager, Karl Chamberlain, reveals the official way to measure a Wotsit crisp as Walkers announces a ??10k reward for whomever finds the biggest Wotsits Giant in the UK.
People travelling outside Waterloo station in London, after Prime Minister Boris Johnson announced a range of new restrictions to combat the rise in coronavirus cases in England. (Photo by Dominic Lipinski/PA Images via Getty Images)
EDITORIAL USE ONLY The Royal Mint has today revealed their latest coin � a Winnie-the-Pooh 50p that has taken the original and well-recognised illustrations by E.H. Shepard of the bear and transformed them onto a UK coin for the first time.
Members of the public are seen by a public information message in central Manchester after Prime Minister Boris Johnson set out new restrictions to last "perhaps six months" to slow the renewed spread of coronavirus, Manchester, England, Tuesday Sept 22, 2020. The UK has reached "a perilous turning point", Boris Johnson said as he set out a raft of new coronavirus restrictions for England. ( AP Photo/Jon Super)
HS2 security guards monitor an anti-HS2 activist standing in the Grand Union Canal to halt tree felling works alongside HOAC lake in connection with the HS2 high-speed rail link on 21 September 2020 in Harefield, United Kingdom. Anti-HS2 activists continue to try to prevent or delay works for the controversial £106bn HS2 high-speed rail link on environmental and cost grounds from a series of protection camps based along the route of the line between London and Birmingham. (photo by Mark Kerrison/In Pictures via Getty Images)
A dancer performs in protest opposite the Central Criminal Court, the Old Bailey, in London, Monday, Sept. 21, 2020, as the Julian Assange extradition hearing to the US continues. (AP Photo/Frank Augstein)
People watch the sun set behind South Stack lighthouse on the Isle of Anglesey, North Wales. (Photo by Peter Byrne/PA Images via Getty Images)
A group of women all wearing witches hats take a selfie.
Stratford-upon-Avon is a market town in the county of Warwickshire, England, on the River Avon. The town is one of the most popular tourist destinations in England as it is the birthplace and gravesite of playwright and poet William Shakespeare. (Photo by Keith Mayhew / SOPA Images/Sipa USA)
Bailiffs and officials leave after serving legal documents in connection with an extended injunction recently granted to HS2 Ltd by the High Court to persons believed to be former inhabitants of a wildlife protection camp occupied by anti-HS2 activists on 21 September 2020 in Denham, United Kingdom. Anti-HS2 activists continue to try to prevent or delay works for the controversial £106bn HS2 high-speed rail link on environmental and cost grounds from a series of protection camps based along the route of the line between London and Birmingham. (photo by Mark Kerrison/In Pictures via Getty Images)
Transport Secretary Grant Shapps arrives in Downing Street, London, ahead of a Cabinet meeting at the Foreign and Commonwealth Office. (Photo by Aaron Chown/PA Images via Getty Images)
People out in Manchester town centre, The latest Public Health England data shows that 672 new cases of Covid-19 were recorded in the city in the seven days to September 17. (Photo by Danny Lawson/PA Images via Getty Images)
Flying Officer James Buckingham salutes The Battle of Britain memorial window inside Westminster Abbey, the stained glass window by Hugh Easton that contains the badges of the fighter squadrons that took part in the Battle, during a service to mark the 80th anniversary of the Battle of Britain at Westminster Abbey, London, Sunday, Sept. 20, 2020. (Aaron Chown/Pool Photo via AP)
89-year-old John Wilkins during his sponsored wing walk for Bristol Children's Hospital's The Grand Appeal at Dunkeswell Airfield near Honiton in Devon. (Photo by Andrew Matthews/PA Images via Getty Images)
PORTSMOUTH, ENGLAND - SEPTEMBER 20: Sophie, Countess of Wessex takes part in the Great British Beach Clean on Southsea beach on September 20, 2020 in Portsmouth, England. (Photo by Pool/Samir Hussein/WireImage)
SOUTHEND-ON-SEA, ENGLAND - SEPTEMBER 20: A man fishes as the sun goes down during recent warm weather on September 20, 2020 in Southend on Sea, London. (Photo by John Keeble/Getty Images)
A man holding a small dog in his arms wears a face covering to help prevent the spread of the coronavirus on 20 September 2020 in Staines-Upon-Thames, United Kingdom. The Borough of Spelthorne, of which Staines-upon-Thames forms part along with Ashford, Sunbury-upon-Thames, Stanwell, Shepperton and Laleham, has been declared an area of concern for COVID-19 by the government following a marked rise in coronavirus infections which is inconsistent with other areas of Surrey. (photo by Mark Kerrison/In Pictures via Getty Images)
Tottenham's Son Heung-min, left, and Tottenham's Harry Kane celebrate at the end of the English Premier League soccer match between Southampton and Tottenham Hotspur at St. Mary's Stadium in Southampton, England, Sunday, Sept. 20, 2020. (Justin Tallis/Pool via AP)
LONDON, ENGLAND - SEPTEMBER 20: A general view of the atmosphere at the Cyberdog X Kappa IRL/VR catwalk to launch the Rave Team Collection during London Fashion Week September 2020 at Cyberdog on September 20, 2020 in London, England. (Photo by David M. Benett/Dave Benett/Getty Images)
Runners and riders in the tote.co.uk Free Streaming Every UK Race Handicap at Chelmsford City Racecourse.
SOUTHEND-ON-SEA, ENGLAND - SEPTEMBER 20: A group of people wearing face masks wait for a bus at the bus stop on September 20, 2020 in Southend on Sea, London. (Photo by John Keeble/Getty Images)
Global Art ridden by Ryan Moore (3rd right) coming home to win the tote.co.uk Free Streaming Every UK Race Handicap at Chelmsford City Racecourse.
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The transition period, which kept the UK aligned to the EU's single market and customs union rules to allow trade to flow smoothly after Brexit, expires at the end of the year unless both sides agree to an extension – something Boris Johnson has ruled out.
The Cabinet Office document states that, in its reasonable worst-case scenario, between 30-50% of trucks crossing the Channel will not be ready for the new regulations coming into force on January 1 2021, while a "lack of capacity to hold unready trucks at French ports" could reduce the flow of traffic across the strait to 60-80% of normal levels.
"This could lead to maximum queues of 7,000 port bound trucks in Kent and associated maximum delays of up to two days," the documents said.
Such delays could be in place for at least three months, hauliers have been warned, as alternative routes are sought and supply chains get to grips with the new systems and requirements.
In his letter, Mr Gove said: "Irrespective of the outcome of negotiations between the UK and EU, traders will face new customs controls and processes.
"Simply put, if traders, both in the UK and EU, have not completed the right paperwork, their goods will be stopped when entering the EU and disruption will occur.
"It is essential that traders act now and get ready for new formalities."
But sector chiefs have accused the Government of failing to do enough in recent weeks over the threat of post-Brexit border delays.
Logistics UK, formerly the Freight Transport Association, was seething last week after being told the Government's Smart Freight system – designed to reduce the risk of cargo delays once Britain is outside of EU rules – would still be in testing mode in January when British exports face new border regulations.
The Road Haulage Association (RHA), meanwhile, said their meeting on Thursday with Mr Gove had fallen "far short of our expectations".
Responding to the worst-case scenario document, RHA chief executive Richard Burnett said: "We've been consistently warning the Government that there will be delays at ports but they're just not engaging with industry on coming up with solutions.
"Traders need 50,000 more customs intermediaries to handle the mountain of new paperwork after transition but Government support to recruit and train those extra people is woefully inadequate.
"The answers to the questions that we raised in our letter to Mr Gove and subsequent roundtable meeting last Thursday still remain unanswered – and our concern continues to grow."
There has been reported action by Government to mitigate the impact around Dover and the south-east ports, however.
There was a suggestion this month a coronavirus testing centre in Ebbsfleet, Kent, was closed to make way for a post-Brexit lorry park to allow customs checks away to be carried out away from the Port of Dover.
A UK Government spokesman said: "With just 100 days to go until the end of the transition period it's vital that businesses prepare now for new rules that will come into force at the end of the year, so that they can hit the ground running on January 1 2021 and seize new opportunities.
"As a responsible Government we continue to make extensive preparations for a wide range of scenarios, including the reasonable worst case.
"This is not a forecast or prediction of what will happen but rather a stretching scenario."
The leaked letter comes as a think tank said failure to reach an agreement with the EU in post-Brexit trade talks could hit Britain's economy three times harder in the long term than coronavirus.
Queues at the border, shortages of fresh food and medicine as well as more "hassle" travelling to the continent are also possible, according to the UK in a Changing Europe group.
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A report by the organisation, based on modelling with the London School of Economics (LSE), said the impacts of coronavirus may mitigate or obscure the effect of a no-deal exit.
But it warned that not forming an agreement with Brussels would have a significant impact in the long term.
The authors wrote: "Our modelling with LSE of the impact of a no-deal Brexit suggests that the total cost to the UK economy over the longer term will be two to three times as large as that implied by the Bank of England's forecast for the impact of Covid-19."
Trade deal talks between the two sides are continuing, but the Prime Minister has set a deadline of October 15 for an agreement to be reached, otherwise he has said he will simply walk away from the negotiating table.