Johnson said Britain should be able to enjoy free trade under an interim post-Brexit agreement with the EU for as long as it takes to negotiate a longer-term trade deal.
He pulled out what some Leave supporters call a "secret weapon" to achieve the seemingly unachievable—a little-known clause in World Trade Organisation (WTO) rules.
Britain's Queen Elizabeth II talks to London Mayor Boris Johnson during the Beijing Olympics Team GB reception at Buckingham Palace in London.
Boris Johnson watches the election results with his wife Marina (L), at the count in Watlington, Oxfordshire, after winning the Henley seat for the Conservatives in the 2001 General Election. The seat was Michael Heseltine's, who has stepped down at this election.
Spectator editor and Conservative MP Boris Johnson (left) chats to former Tory Chancellor Lord (Norman) Lamont at a party to mark the Spectator's 175th anniversary, at the Four Seasons Hotel, Park Lane, London. Among those also in attendance at the party were Home Secretary David Blunkett, broadcasters David Dimbleby and Andrew Neil, and TV chef Nigella Lawson.
Boris Johnson, Conservative MP and editor of The Spectator magazine, at a party to mark The Spectator's 175th anniversary, at the Four Seasons Hotel, Park Lane, London. Among those also in attendance at the party were Home Secretary David Blunkett, broadcasters David Dimbleby and Andrew Neil, and Ulster Unionist leader David Trimble. 16/10/2004 The Spectator editor Johnson, has been ordered by Tory leader Michael Howard to go to Liverpool to apologise for an article in his magazine accusing its people of 'wallowing' in their 'victim status'. Mr Johnson, who is also the Conservative shadow minister for the arts, said he would be travelling to the city next week in a 'spirit of complete humility' to apologise in person for the offence caused.
Shadow Arts Minister Boris Johnson campaigns for donations from delegates at the Conservative Party Annual Conference in Bournemouth. 15/10/2004: Tory MP Boris Johnson who edits The Spectator magazine where a leading article has accused Liverpudlians of wallowing in what they regard as their "victim status" over the murder of Ken Bigley and the Hillsborough football disaster in 1989.
Conservative Party MP Boris Johnson sits through a series of speeches during the Conservative Party Conference in Bournemouth.
Boris Johnson MP (centre) and editor of the Spectator at the Liverpool Institute for Performing Arts during a visit to the city to apologise for remarks made in his magazine about beheaded hostage Ken Bigley and the Hillsborough tragedy. But council leaders called on the people of Liverpool to ignore the "Boris Johnson show". The Tory MP for Henley created a storm of protest after the publication of a leader written in the Spectator, which suggested Liverpudlians were "hooked on grief".
Boris Johnson MP and editor of the Spectator magazine arrives at Claridges Hotel in London, where he is hosting the 'Spectator Parliamentarian of the Year' Awards. 13/11/2004 Boris Johnson who was Saturday November 13 2004, sacked from the Conservative frontbench amid fresh allegations about his private life, a spokesman for Tory leader Michael Howard said.
Boris Johnson arrives at his house in north London, after going for an early morning jog. The former Shadow Arts Minister and Deputy Chairman of the Conservative Party, was relieved of his position by party leader Michael Howard, after revelations about his private life were published in a tabloid newspaper.
Boris Johnson, MP for Henley and Conservative spokesman on Higher Education, opens a footpath at the Environments Agency in Wallingford, Oxon, Friday April 7 2006. He later started a race between a car, a cyclist, runners and walkers to promote green travel. Watch for PA story.
England's Boris Johnson and Germany's Maurizio Gaudino shake hands at the end of the game
England's Boris Johnson (L) tackles Germany's Maurizio Gaudino during the Legends match at the Madejski Stadium in Reading.
Shadow education spokesman Boris Johnson MP, attends the final day of the Conservative Party's Annual Conference in Bournemouth.
Boris Johnson MP addresses a group of sixth form students from schools across London after being handed a petition against plans to scrap the last remaining ancient history A-level outside the House of Commons, London.
Boris Johnson (left) joins the Save our Services campaign against health cuts at St, Mary's Hospital in Sidcup, Kent.
Candidates for Mayor of London, Labour's Ken Livingston (left), Lib Dem candidate Brian Paddick (centre) and Boris Johnson of the Conservative party, appear together on television debate.
Conservative candidate for Mayor of London Boris Johnson launches the final phase of his mayoral campaign at Bounces Road Community Hall, North London today.
Conservative Party leader David Cameron congratulates Boris Johnson on becoming London Mayor at Tory HQ in central London.
Newly elected London Mayor Boris Johnson wears a policeman's hat in Trafalgar Square, central London during the Sikh New Year Festival celebrations of Vaisakhi.
Mayor of London Boris Johnson waves the Olympic Flag during the handover ceremony as President of IOC Jacques Rogge (centre) and Mayor of Beijing Guo Jinlong look on at the National Stadium during the 2008 Beijing Olympic Games, China.
Retired Formula 1 World Champion Mika Hakkinen (right) and current Formula 1 World Champion Lewis Hamilton (left) join Mayor of London Boris Johnson in backing an anti-drink-drive campaign at Potters Fields Park in London.
London Mayor Boris Johnson (2nd Left) celebrates St Georges Day in Leadenhall Market in the City of London today where he saw a festival of English food and drink.
Leader of the Conservative Party David Cameron (2nd right) with Mayor of London Boris Johnson (2nd left) as they celebrate St George's day in Leadenhall Market in the City of London.
Boris Johnson and TV star Kelly Brook launch the Mayor of London's Skyride, Peter's Hill steps, London.
London Mayor Boris Johnson with a sculpture of a blue cockerel as he unveils the next two commissions for the fourth plinth in Trafalgar Square, which includes the cockeral (pictured) by Katharina Fritsch and a boy on a rocking horse named Powerless Structures by Michael Elmgreen and Ingar Dragset.
Boris Johnson pictured with members of the Genesis Mas band, who will be performing at the Notting Hill Carnival this weekend, at Potters Fields in central London.
Mayor of London Boris Johnson (left) with Prime Minister David Cameron during the lighting of the Paralympic Cauldron in Trafalgar Square, central London.
London Mayor Boris Johnson boards one of the first trains of the new London Overground extension connecting Clapham Junction with Canada Water. ASSOCIATION Photo. Picture date: Monday December 10 2012. The new service will be London's first new orbital Journey times from south and east London. See PA story RAIL Link. Photo credit should read: Stefan Rousseau/PA
Actor Brian Blessed (left) and Mayor of London Boris Johnson sell poppies in Liverpool Street Station in central London with they collected money from commuters including nine year old Arthur Crowley (centre) from Salisbury.
Mayor of London Boris Johnson (centre) swings from a bus as Northern Ireland Secretary Theresa Villiers (left), Northern First Minister and DUP leader Arlene Foster and Minister of Enterprise, Trade and Investment Jonathan Bell (second right) look on during a visit to Wrightbus Chassis plant in Antrim.
Foreign Secretary Boris Johnson (left) and his Russian counterpart Sergei Lavrov during a press conference following their meeting in Moscow.
WTO rules generally prevent countries agreeing to have no tariffs, as Britain and the EU would do under Johnson's plan, unless they scrap tariffs for every other country too.
But Article 24 creates an exception for countries which have signed free trade agreements with each other, or have a detailed plan and timetable to create one.
This is how the EU itself is allowed to keep frictionless trade and zero tariffs among members, while keeping certain barriers to other countries outside the bloc.
And this is also the tool Johnson, Farage and other Brexiteers hope to use to keep free trade with the EU immediately after Brexit, without having to immediately open Britain up to competition from the whole world too.
"There will be no tariffs; there will be no quotas because what we want to do is get a standstill in our current arrangements under GATT 24—or whatever it happens to be—until such time as we have negotiated an FTA," Johnson said on the 'Our Next Prime Minister' debate.
Why are there major holes in the plan?
So far so good—or so it seems. UK prime minister Theresa May is not the first to have pointed out the apparent magic bullet "does not actually reflect accurately" Britain's situation.
The first problem is that the EU itself would have to agree to form this kind of 'interim agreement' to get it off the ground.
It is far from clear the EU would be happy to grant Britain such relatively generous terms for up to 10 years after the UK leaves, particularly before any future relationship had been agreed.
The second problem is that Britain and the EU are only allowed an interim agreement if they can prove they already have a "plan and schedule" towards a future free trade deal. If not, other WTO countries have a right to demand they scrap the plans.
Johnson suggested he could keep short-term free trade while negotiating a long-term deal - but he would actually have to negotiate the future deal before he could guarantee immediate free trade.
Could Johnson pull off a deal before Brexit?
That would give the government and Brussels until 31 October to successfully negotiate large chunks of their future relationship, which is a seemingly impossible deadline.
Many free trade deals take years to negotiate, and the EU is unwilling to negotiate the future relationship until Britain's divorce is settled. Johnson's recent threat to withhold Britain's divorce bill could make negotiations even harder.
The EU's trade commissioner Cecilia Malmstrom called the Article 24 proposal "completely wrong" earlier this month, saying Britain would inevitably face new tariffs if it left without a deal.
As Peter Ungphakorn, a trade expert and former WTO official, has noted: "That's a pretty blunt 'secret weapon.'"