Brexit figurehead Boris Johnson has declared he will vote for Theresa May’s EU Withdrawal Agreement in a special sitting of the House of Commons on the day the UK was scheduled to leave the European Union.

Mr Johnson said he had taken the “painful” decision to back a deal he once claimed would leave the UK as a “vassal state” because defeat for the Prime Minister would risk “an even worse version of Brexit or losing Brexit altogether”.

Today is the day we promised the country we would leave the European Union. I bitterly regret that we have failed to keep that promise. Instead we will today vote on the Withdrawal Agreement — Boris Johnson (@BorisJohnson) March 29, 2019

The move boosts Mrs May’s chances of securing a majority in the Commons for the Withdrawal Agreement which she struck with the remaining 27 EU states last November.

READ MORE: David Mundell accuses MPs of ‘politicking’ over Brexit deal

After Mr Johnson declared he would be backing the deal, First Minister Nicola Sturgeon responded on Twitter, branding him a "shameless charlatan".

She said: "Alert - this shameless charlatan, devoid of any principle whatsoever, is on the loose. Might be accompanied by a pal called Jacob."

Alert - this shameless charlatan, devoid of any principle whatsoever, is on the loose. Might be accompanied by a pal called Jacob. https://t.co/psabAMZo9d — Nicola Sturgeon (@NicolaSturgeon) March 29, 2019

In a special sitting of the Commons, MPs are debating the legally binding treaty segment of the Prime Minister’s Brexit deal, but not the accompanying Political Declaration setting out plans for the future EU/UK relationship.

The PM’s decision to split the two parts of the package means it is not a third attempt to pass a “meaningful vote” on the Government’s Brexit deal and complies with rules laid down by Commons Speaker John Bercow.

Under the terms of an agreement with Brussels, if passed by MPs on Friday the vote would qualify the UK to be granted an automatic delay to May 22 of the formal date of Brexit.

There were signs that Mrs May’s gambit has succeeded in winning over some of the Conservative rebels who voted against her deal in “meaningful votes” when it went down to defeat by a margin of 230 in January and 149 in March.

READ MORE: How is this Commons Brexit vote different from before?

Former chief whip Mark Harper and European Research Group members Lucy Allan and Daniel Kawczynski were among Eurosceptic backbenchers announcing they would vote with the Government. ERG chairman Jacob Rees-Mogg said he would “reluctantly” back it, despite previously saying he would vote with the DUP.

And Mr Johnson wrote on Twitter: “I have been and remain intensely critical of the deal. But we have a choice to make now, and that means choosing between options that actually exist.

“I have come to the sad conclusion that neither this Government nor this Parliament is willing to leave with no deal. We therefore run the risk of being forced to accept an even worse version of Brexit or losing Brexit altogether.

“A bad deal that we have a chance to improve in the next stage of negotiations must be better than those alternatives.

“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.”