THE countdown to Theresa May’s departure as Prime Minister has begun and she could announce her resignation from office in just three weeks’ time if, as many expect, her fourth and final attempt to get her Brexit plan through the Commons fails.

Ministerial sources suggested even if she won the crunch vote on the Withdrawal Agreement Bill[WAB] in early June, raising the prospect she might secure a deal, Mrs May would still agree to step down by the end of July, sparking a Tory leadership contest.

As events unfolded at Westminster, by coincidence or design, Boris Johnson announced his intention to throw his hat in the ring to become Britain’s new Prime Minister once Mrs May said she was standing down.

“Of course, I'm going to go for it," the former Foreign Secretary told a business event in Manchester.

While the PM did not name a date for her departure as many Tories had hoped, she agreed to set a timetable for the election of her successor.

However, Mrs May is not in control of events.

One senior minister told The Herald if the European election results for the Conservatives were as bad as feared, then a delegation of Cabinet ministers could visit the PM over the Whitsun recess to “put her out of her misery and tell her the game is up”.

He explained: “Why would you put Theresa through another humiliating parliamentary defeat?”

Ian Blackford for the SNP mocked the PM, saying she was "so incompetent she can’t even resign properly".

In the Commons, Mrs May was given a reprieve at an “emotional” meeting of the executive of the Tories’ 1922 backbench committee. Sir Graham Brady, its Chairman, admitted there had been a “frank exchange” of views.

At one point, Mrs May was said to have been “very teary” as she argued for more time to have one last go at getting her deal through the Commons.

After a 90-minute meeting the party grandees agreed. The PM will now meet Sir Graham following the WAB vote. He explained this would “decide the timetable for the election of a new leader…regardless of what the vote is on the Second Reading; whether it passes or whether it fails".

One minister was clear, telling The Herald: “If the WAB is lost, we are into resignation territory and a leadership election to follow.”

A Tory MP said: “It’s been over for some time. I wanted the PM to be able to leave office with the dignity and respect she undoubtedly deserves. I can’t understand why she hasn’t take the path she has been offered.”

Another backbencher claimed Downing St was responsible for the Tory turmoil. “No 10’s bunker mentality has got us here.”

A Downing St insider acknowledged if the WAB were defeated, the pressure for Mrs May to go immediately would ratchet up.

Given the number of potential candidates, nine, including Michael Gove, Jeremy Hunt and Dominic Raab, any contest is likely to run through July and even into August with the new leader setting out their stall at the Tory conference in September.

One possible hope for Mrs May would be if Labour ordered its MPs to abstain rather than vote against the WAB. This would enable Jeremy Corbyn to try to amend the legislation to make it more in line with his party’s thinking.

However, the party leadership appears to be split on what course to take.

On Wednesday, the Labour leader’s spokesman conspicuously failed to say Labour would oppose the passage of WAB. Hours later, Emily Thornberry, the Shadow Foreign Secretary, insisted it would.

Yet when Mr Corbyn was asked if this was the case, he stopped short of confirming Labour would vote against the bill, saying only his party would “not support it" unless the legislation met its demands.

He said: "We've put to the Government a very strong position that we want a customs union with the European Union to protect jobs and trade, as well as trade access, and we want to protect the rights at work, consumer rights and environmental rights.

"Our view is there has to be a relationship with Europe, which guarantees those things and if that bill doesn't do it, then we won't support it."