May says her withdrawal plan is the best way to protect the union, but opposition parties and the Democratic Unionist Party (DUP) say it could have the opposite effect.
Those assurances, the withdrawal agreement and political declaration on the future EU-UK relationship had ‘legal force in international law’ May said.
The backstop proposals are designed to prevent a hard border in Ireland in the event that the UK and EU cannot agree a future trade deal.
But opponents say that if it came into force it would lead to extra checks for some goods coming into Northern Ireland only and threaten the integrity of the union.
The DUP, which keeps the minority government in power through a ‘confidence and supply’ agreement, said their 10 MPs would still vote against the withdrawal deal tomorrow.
Their Westminster leader Nigel Dodds dismissed the letters, saying ‘nothing had fundamentally changed’ since a December vote on the deal was postponed.
He told BBC’s Good Morning Ulster that the letters were not legally binding, even though the Prime Minister has said they would be when she delayed the vote.
“It’s another example that the EU is not prepared to do what’s required, even to take the first step if it wants to get a deal in the House of Commons,” he added.
The DUP and a number of Tory Brexiteers want a binding pledge from the EU27 that the backstop would be temporary at the very least.
They would prefer to see negotiations re-opened with the backstop removed entirely, or its terms dramatically changed.
EU leaders say they will not agree to anything that alters the legally binding withdrawal agreement, which they approved last year.
Up to 100 Tory MPs are set to join Labour and other opposition parties to vote against the Prime Minister’s deal in the Commons tomorrow night.