Most of the 27 European leaders gathered in Brussels for Brexit crisis talks, including German Chancellor Angela Merkel, backed a plan for Brexit to be postponed for up to a year.

But as the talks went late into the night, Macron -- with backing from Belgium, Austria and some smaller EU states -- held out for a short delay of only a few weeks and demanded solid guarantees that London would not interfere in EU business during that time.

An official from the French presidency insisted that France was not isolated, but added: "If we are more insistent than the others, so be it. We can't allow the good functioning of the EU to be disrupted in the weeks to come."

May has already said that if Britain is still an EU member when the European parliamentary election begins on May 23, UK voters will take part. But some EU leaders are unconvinced that she is sincere, despite one official telling reporters her presentation had been "solid".

After May made her case for an extension until June 30, she left the other 27 EU leaders to ponder Britain's fate over a three-course dinner at the EU Council headquarters.

Without a postponement, Britain is due to end its 46-year membership of the European Union at midnight (2200 GMT) on Friday with no deal, risking economic chaos on both sides of the Channel.

European Council president Donald Tusk, the summit host, has proposed "a flexible extension" and Merkel said EU leaders may well back a delay "longer than the British prime minister has requested".

But, as he arrived, Macron warned: "For me, nothing is decided, nothing, and in particular, since I hear rumours, not a long extension."

He repeated his insistence that May must provide more guarantees that the delay would serve a useful purpose, saying he wanted to hear "what is the political plan behind it".

May agreed a divorce deal with the EU last November but MPs in London have rejected it three times, forcing her to turn to the main opposition Labour party in a bid to find a way through.

But these talks are moving slowly, and the prime minister is under intense pressure from hardline Brexit supporters in her Conservative party not to compromise.