The UK will face an "economic war" with Europe and America if Boris Johnson fails to negotiate a Brexit deal, Gordon Brown has warned.
The former prime minister said Mr Johnson's confrontational approach will make him the "most isolated prime minister in peacetime history" in the event of a no-deal Brexit.
Negotiations between British and EU negotiators are set to resume today in Brussels in an attempt to secure a post-Brexit trade deal.
Warning of "huge international implications" if no agreement is reached, Mr Brown said: "We would be in an economic war with Europe that would cost us very dearly.
"Food, drugs and everything else would find it difficult to get into the country without tariffs and without holdups.
"But we'd also be in an economic war with America because there'd be no chance of a trade treaty with America.
"So Boris Johnson is going to end up as the most isolated Prime Minister in peacetime history, with no friends around the world.
"Because he has simply chosen a path of confrontation when everybody knows that it's in Britain's economic interests – maybe not in the Brexiteers' ideological interest – but it's in Britain's economic interest to get a deal and to get a deal now."
Cherie Blair, wife of newly elected Labour Party leader, Tony Blair, celebrates with former leader, Neil Kinnock, and Gordon Brown at the Institute of Education.
The new Shadow Cabinet in the Shadow Cabinet Room at the House of Commons.
From left, Back row: Michael Meacher, Dr David Clarke, Donald Dewar, Bryan Gould, Barry Jones, Tony Blair and Frank Dobson.
Front: Dr John Cunningham, Derek Foster, Jo Richardson, Stan Orme, Gerald Kaufman, Neil Kinnock, Roy Hattersley, Robin Cook, John Prescott, Gordon Brown and Jack Straw.
Labour leader Neil Kinnock with his economic team in London, where the Shadow Cabinet was meeting.
Shadow Home Secretary and Labour Leadership front runner Tony Blair confers with Shadow Chancellor Gordon Brown outside the House of Commons.
Labour Party leader Tony Blair (R) and deputy leader Gordon Brown during a preview of Labour's pre-budget party political broadcast at Church House, Westminster, London.
Labour leader Tony Blair, flanked by Gordon Brown (left) and deputy leader John Prescott, unveils the party's plans to prepare for a general election, during a news conference by the National Executive Committee, in London.
Chancellor of the Exchequer Gordon Brown meets Pain (a character from the film Hercules) when Mr Brown hosted a Christmas party to benefit SCOPE at No.11 Downing Street this evening.
Chancellor of the Exchequer and Chairman of the Committee for the Diana, Princess of Wales Memorial Fund, Gordon Brown and Diana's sister, Lady Sarah McCorquodale, speak to the media outside No. 11 Downing Street today (Monday) following the first meeting of the committee this afternoon. Watch for PA Story.
(l/r) British Prime Minister Tony Blair, Deputy Labour Party leader John Prescott and Chancellor of the Exchequer Gordon Brown listen intently to David Obaje speaking on the platform at the Labour Party Conference at the Winter Garden conference Centre in Blackpool this morning.
Prime Minister Tony Blair (L) and Chancellor of the Exchequer Gordon Brown, during the Nicky Campbell Show, a live phone-in programme at the BBC Radio 5 Live studios in Central London.
File photo of Prime Minister Tony Blair (left) and Chancellor Gordon Brown sampling a cup of coffee at the Starbucks Coffee Bar in Villers Street, central London, dated 10/11/99.
Chancellor Gordon Brown, 49, and his wife, PR executive Sarah Macaulay, 36, after their wedding in a small private ceremony held at his home in North Queensferry, Fife. The couple enjoyed a 6 year courtship before their wedding.
Britain's Prime Minister Tony Blair flanked by Chancellor Gordon Brown (right) and Education Secretary David Blunkett sit in a first class coach on to a Virgin service at London's Euston Station for the one-and-a-half-hour trip to Birmingham, for the party's manifesto launch.
Britain's Prime Minister Tony Blair (left) and Chancellor of the Exchequer Gordon Brown on the Labour "battle bus" on the way to Harlow, Essex, where Mr Blair appealed to mainstream Conservatives to back the Labour Party in the General Election.
Prime Minister Tony Blair (left) and Chancellor Gordon Brown during a pre-election visit to a fish and chip shop in Harrow, north London.
Chancellor of the Exchequer Gordon Brown is given a cup of coffee by his wife Sarah in his Downing Street apartment as he prepares to deliver his 2002 budget later. Brown is expected to announce an increase in National Insurance contributions. *... to help pay for the rejuvenation of the National Health Service.
Prime Minister Tony Blair, The Chancellor of the Exchequer Gordon Brown (left) and Health Secretary Alan Milburn (right) in the House of Commons during Prime Ministers' Questions in London. * Downing Street said today that Mr Blair remained "confident that a second United Nations resolution paving the way for war with Iraq would be carried. The comments came after Mr Blair held 50 minutes of talks in No 10 with Russian foreign minister Igor Ivanov this morning.
The Chancellor of the Exchequer Gordon Brown leaves Number 11 Downing Street to present his seventh Budget to the House of Commons, central London.
The Chancellor Gordon Brown (centre right) holds a torch lit in Athens as it arrives in London's Parliament Square on its way to Glasgow and then Dublin, for the Special Olympics World Summer Games which are being held in the city from 21 June.
Chancellor Gordon Brown chats to players before starting the charity football matches taking place in Victoria Tower Gardens, central London to raise money for the charity Piggy Bank Kids, which was started by Brown and his wife Sarah.
Chancellor Gordon Brown, 52, holds his son John as he stands with his wife Sarah, 38, at the Edinburgh Royal Infirmary. The Chancellor's newborn son, John Brown was born on Friday 17th October 2003. The new parents were beaming with joy as they posed for photographs before heading off to their home in North Queensferry.
Rt Hon Gordon Brown MP with tv presenter Fiona Phillips during the ePolitix Charity Champion Awards at the QE2 Conference Centre in Westminster, central London. Gordon received the Children and Youth Champion Award at the ceremony which pays tribute to the hard work and time key parliamentarians devote to charitable causes and capaigns in Parliament.
Chancellor Gordon Brown chats with Radio One DJ Sara Cox, at 11 Downing Street, during a Christmas party the Chancellor hosted for the children's cancer and leukaemia charity, CLIC, which over two decades has been transformed into one of Britains most dynamic children's charities, offering help and support to a growing number of families across the UK.
Gordon Brown, Chancellor of the Exchequer is presented with a shirt from Charlton Athletic's Manager Alan Curbishley
Chancellor Gordon Brown (left) stands with Dr Alan Greenspan, Chairman of the US Federal Reserve.
Sir Bob Geldof (right) and British Chancellor Gordon Brown at the British Museum in central London, where they met members of the Africa Children's Choir, who were there as part of Africa Live - a venture between the museum and the BBC.
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Mr Brown said it would be up to Labour leader Sir Keir Starmer whether to back any deal but suggested "most of the big questions" are likely to remain unanswered even if an agreement is reached, leading to "an unstable relationship" over the coming months and years.
He predicted that there will be a tariff-free trade deal without quotas but said the EU may insist upon sanctions and tariffs being imposed if the UK breached the "level playing field".
"So you may have an unstable relationship over the next few months or even over the next few years because all they can get to in this negotiation is a minimalist deal, which leaves people very uncertain about the path of the economy", Mr Brown said.