Britain's economy will shrink if the country leaves the European Union next year without a Brexit deal.
That's according to the IMF which also said any deal will leave Britain financially worse off than staying in the EU.
The IMF said it expected Britain's economy would grow by about 1.5% a year in 2018 and 2019 if a broad Brexit agreement was struck, compared with about 1.75% if it had stayed in.
Failure to get a deal would lead to a contraction, IMF Managing Director Christine Lagarde said.
"A more disruptive departure will have a much worse outcome," she said as the IMF presented its annual report on Britain'seconomy.
"Let me be clear, compared with today's smooth single market, all the likely Brexit scenarios will have costs for the economy and to a lesser extent as well for the EU," she said.
"The larger the impediments to trade in the new relationship, the costlier it will be. This should be fairly obvious, but it seems that sometimes it is not."
Britain is due to leave the EU in March next year but London and Brussels have yet to strike a deal to secure a transition period.
Prime Minister Theresa May is hoping to make progress towards a deal when she meets fellow EU leaders this week.
The IMF said there was a "daunting" range of issues still to be dealt with before Brexit.
Britain's economy - the world's fifth-biggest - slowed after the 2016 referendum decision to leave the EU and it continues to be outpaced by most other rich nations.
However, stronger-than-expected data last week showed the economy had its fastest growth in nearly a year, helped by the World Cup and hot summer weather.
British finance minister Philip Hammond, speaking alongside Ms Lagarde, said the government had to heed the "clear warnings" from the IMF of a no-deal Brexit.
Hammond has been criticised by some Brexit supporters who say he wants to maintain a relationship with Brussels that would keep Britain under too much influence from the EU.
'Good cooperation' in talks; Barnier
Talks between the European Union and Britain on Brexit are being conducted in a spirit of "good cooperation", according to Michel Barnier said.
Mr Barnier, the EU's chief Brexit negotiator, was in Madrid to brief Spanish Prime Minister Pedro Sanchez and Foreign Minister Josep Borrell on the progress in the Brexit negotiations.
A Spanish government source saidBarnier would also discuss Gibraltar, a British territory on Spain's southern coast which will leave the EU along with the UK on March 29th next year.
"We continue working in a spirit of good cooperation," Mr Barnier told reporters as he left the Foreign Ministry.
Despite Barnier's upbeat mood, there is, so far, no full exit agreement and some rebels in British Prime Minister Theresa May's Conservative Party have threatened to vote down a deal if she clinches one with the EU.
A major obstacle facing Britain as it negotiates its exit from the EU, including the single market and customs union, is future arrangements for the border between the Irish Republic and Northern Ireland, which is part of the UK.