The Prime Minister has insisted that she has secured “legally binding” changes to the Brexit deal which ensure the Irish backstop cannot be permanent.
Following last-minute talks in Strabourg ahead of the vote today, Mrs May has said she “passionately believed” her Brexit deal addressed concerns raised by MPs who feared the backstop would keep the UK in a customs arrangement with the EU indefinitely.
At a joint press conference with European Commission president Jean-Claude Juncker, Mrs May said the three new documents agreed provided the legal assurances critics of her stance had called for.
The Prime Minister said: "What we have secured is very clearly that the backstop cannot be indefinite. Cannot become permanent. It is only temporary. If it is the case that we were ever to get into the backstop.
"The legal instrument that we have agreed is an addition to the Withdrawal Agreement. It has the same legal status as the Withdrawal Agreement. It is legally binding.
"That is what Parliament asked us to secure and that is what we have secured."
'No third chance'
The European Commission president insisted there would be no further negotiations on the issue.
Mr Juncker said: "There will be no new negotiations. It is this.
"In politics, sometimes you get a second chance. It is what we do with the second chance that counts. Because there will be no third chance.
"Let us be crystal clear about the choice - it is this deal or Brexit might not happen at all."
Mrs May believes the three new documents agreed with Mr Juncker will give MPs the assurances to approve her Withdrawal Agreement and Political Declaration on the future EU/UK relationship.
The government has said that one document "reduces the risk that the UK could be deliberately held in the Northern Ireland backstop indefinitely" and another was described as a "unilateral declaration by the UK" which sets out "the sovereign action the UK would take to provide assurance that the backstop would only be applied temporarily".
If these new documents are passed by MPs, leaders of the 27 remaining EU states will be asked to endorse the new documents at a scheduled European Council summit in Brussels on March 21.
'Failed to effectively negotiate'
The DUP has pledged to carefully analyse the Brexit deal add-ons, remaining non-committal on whether it would now back the Withdrawal Agreement.
Deputy leader Nigel Dodds told the Commons "all of this will need to be taken together and analysed very carefully".
But Labour leader Jeremy Corbyn dismissed the move and called on MPs to reject the deal.
He said: "The Prime Minister's negotiations have failed.
"This evening's agreement with the European Commission does not contain anything approaching the changes Theresa May promised Parliament, and whipped her MPs to vote for.
"Since her Brexit deal was so overwhelmingly rejected, the Prime Minister has recklessly run down the clock, failed to effectively negotiate with the EU and refused to find common ground for a deal Parliament could support."