Anticipation of May's offer comes as British parliament tries to select its own alternative from a multiple-choice list of Brexit options.

As the United Kingdom's three-year Brexit crisis spins towards its finale, it remains uncertain how, when or even if it will leave the European Union, though May hopes to bring her deal back to parliament later this week.

With British politics at fever pitch, lawmakers on Wednesday grab control to have so-called indicative votes on Brexit, with options ranging from a much closer post-exit alignment with the EU to leaving without a deal or revoking the divorce papers.

Just two days before the United Kingdom had been originally due to leave the EU on March 29, some of the most influential Brexit-supporting rebels, such as Jacob Rees-Mogg, have now reluctantly fallen in behind May's deal.

The price for May may be her job.

She is expected to indicate a date for her departure at a showdown with Conservative Party lawmakers at a meeting of the 1922 Committee in Westminster about 5pm London time (4am Thursday AEDT).

Before that, lawmakers start a debate on what sort of EU divorce the world's fifth largest economy should opt for, before voting on a ballot paper for as many proposals as they wish. Results will be announced after 9pm (8am Thursday AEDT).

"The prime minister might get a deal over the line on Thursday or Friday," said Oliver Letwin, a Conservative former cabinet minister who has led parliament's unusual power grab. "If she does, no one would be happier than I am."

"If, however, that doesn't happen and if we do go forward to Monday, and if on Monday one or more propositions get a majority backing in the House of Commons, then we will have to work with the government to get the government to implement them."

The uncertainty around Brexit, the United Kingdom's most significant political and economic move since World War II, has left allies and investors aghast.

Opponents fear Brexit will divide the West as it grapples with both the unconventional US presidency of Donald Trump and growing assertiveness from Russia and China.

Supporters say while the divorce might bring some short-term instability, in the longer term it will allow the United Kingdom to thrive if cut free from what they cast as a doomed experiment in European unity.

May's deal, an attempt to soothe the divide of the 2016 referendum by leaving the formal structures of the EU while preserving close economic and security ties, was defeated in parliament by 149 votes on March 12 and by 230 votes on January 15.

Brexit supporters fear the entire divorce is at risk. The government could try to ignore the votes, though if May's deal fails then an election could be the only way to avoid parliament's alternative proposal.

Australian Associated Press