March 29 (UPI) -- On the day Britain was supposed to exit the European Union, lawmakers in London instead Friday will vote for a third time on Prime Minister Theresa May's Brexit plan to leave from the 28-nation alliance -- hoping for an extended deadline to depart.

May told lawmakers this week she would resign if they backed her plan and the European Union has proposed an extended deadline, May 22, to leave. The EU said the extension, however, can only be granted if lawmakers pass the main part of May's proposed agreement -- and it must be passed Friday.


If the agreement is not passed, the new deadline to leave will be April 12.

May's proposal includes two documents -- the withdrawal agreement and a political declaration. Lawmakers need only to pass the withdrawal agreement to get the May 22 deadline, the EU said Friday.

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House of Commons speaker John Bercow initially blocked a third vote, but said May's dropping the political declaration from the bill made enough difference to put it up for a vote again -- angering some Labor Party leaders who've been pushing for a second public referendum.

The late negotiations have achieved some movement. Brexit supporter Boris Johnson said Friday he will vote for May's plan after weeks criticizing it.

"It is very painful to vote for this deal. But I hope we can now work together to remedy its defects, avoid the backstop trap and strive to deliver the Brexit people voted for," he tweeted.

Conservative Party lawmakers Bill Cash and John Redwood said they are still against the deal, largely due to the Irish backstop issue, which Cash said "drives a coach and horse through our precious union."

British Parliament has already voted twice against May's proposal, which she negotiated with the European Union. The prime minister later made several attempts to renegotiate with the EU after lawmakers in London overwhelmingly rejected the plan. The European Union said it's prepared for Britain to leave without a deal in place, a prospect some worry could have severe economic consequences. A number of countries have said they might pull operations out of Britain if it leaves the EU without a trade agreement.

Hundreds of thousands of Britons marched in London last weekend calling for a second referendum on the issue -- basically a re-vote to determine whether they really want to leave the 28-nation alliance. The first was held in mid-2016.