European Union leaders should be patient with Britain over its struggle to find majority support in parliament for a deal to exit the bloc, EU leaders group chairman Donald Tusk said on Tuesday.

He was reacting to Prime Minister Theresa May's statement saying she would ask the EU for a further delay to the day of Brexit and would sit down with opposition Labour leader Jeremy Corbyn in an attempt to break the impasse in parliament.

"Even if, after today, we don’t know what the end result will be, let us be patient," Tusk said on Twitter.

Danish Prime Minister Lars Lokke Rasmussen cautiously backed Tusk but expressed doubt whether May's attempt for a cross-party compromise on Brexit, something EU officials have said May should have pursued three years ago, was to be taken seriously.

"Since we could agree to postpone Brexit to right before European Parliament election (on May 23-26) given the approval of May’s agreement, we should also be patient IF there suddenly is a cross party way forward in UK. But is it to good to believe?" Rasmussen said on Twitter.

May said the extension she would seek beyond the current legal departure day of April 12 should be "as short as possible" without specifying any period, and added that it should end when a deal is ratified by parliament.

She did not make clear what kind of solution she would seek together with Corbyn, who supports the option of Britain staying in a customs union with the EU - something May's party rejects.

EU officials who asked not to be identified said it was not clear what compromise May was trying to achieve and that was why Tusk was calling for patience.

But patience has limits, the EU officials said, because on April 10 EU leaders will meet in Brussels for a summit to finalise Brexit. April 12 has already been set as the date on which Britain will leave the EU without a deal, unless a request for an extension is approved by EU leaders.

"I'm not sure what her game plan is or where it is headed," one EU official closely involved in the Brexit process said.

"If it’s for real that there’s now a cross-party process in the UK, everybody here will applaud it. If it actually can go somewhere, with real engagement on both sides, I think leaders would look favourably at possibilities of an extension.

"If it’s a Hail Mary because all other options are closed - because parliament is all no-no-no-...and the cabinet (is) just as split as the parliament - then I think EU leaders will ask themselves if we’re not at the end of the road," the official added.