May has embarked on a last-ditch battle to postpone Brexit from April 12 to June 30 to arrange an orderly departure -- but European leaders are expected to offer her a longer delay of up to a year.

"There are times when you need to give time," EU Council President Donald Tusk tweeted, as he issued summit invitations.

Summit host Tusk said the evidence of recent months gave EU leaders "little reason to believe" that British lawmakers will ratify the Brexit withdrawal treaty before May's preferred June 30 departure.

"In reality, granting such an extension would increase the risk of a rolling series of short extensions and emergency summits, creating new cliff-edge dates," he said, reflecting concern in EU capitals.

"One possibility would be a flexible extension, which would last only as long as necessary and no longer than one year," he said.

According to a draft copy of the summit conclusions that EU leaders were to receive and debate later in the day, they will agree to an extension to allow May time to ratify the withdrawal agreement.

Britain was originally due to leave the EU on March 29, but Brussels agreed to an extension after the British parliament rejected the withdrawal agreement negotiated with May.

The British premier is hoping a new period of extra time, if granted, will enable her to finally get a divorce deal through the legislature.

British MPs have rejected her deal three times, but May is now in talks with the opposition Labour party to try to break the deadlock.

These discussions are moving slowly, and EU negotiator Michel Barnier said May must explain what postponement would achieve.

AFP