An MP’s attempt to add an end date to the Irish border backstop in Theresa May’s Brexit deal has raised speculation the government could use it to try and wring fresh concessions from the EU.

Tory Andrew Murrison’s amendment backs the prime minister’s deal subject to a “legal codicil” adding an December 31, 2021 expiry date to the controversial backstop.

The amendment will have no legal effect unless the EU agrees to rewrite the text of the withdrawal agreement, which it has so far refused to do.

But there are reports that the government could urge MPs to back the move to show that May’s deal can pass through the Commons if the EU listens to demands for an end date.

It could also be a way for May to avoid a landslide defeat in Tuesday’s crunch vote on her deal, as it could win the support of Tory opponents who fear the UK being trapped in the backstop’s customs union with the EU permanently, severely restricting the government’s ability to strike free trade deals around the world.

Both Government sources and Murrison insisted the move was not being pushed by government whips.

However, ultra-loyal Tory MP Robert Goodwill told HuffPost UK he had signed Murrison’s amendment as it could act as a “signpost”, fuelling speculation the government could back it in a vote.

Murrison, who chairs the Commons Northern Ireland committee and announced over the weekend he would back the deal, told the Press Association: “It is important in my opinion that we understand the backstop is not needed to ensure the absence of a hard border in Northern Ireland.

“The amendment I am tabling will address the concerns I have heard expressed by a large number of colleagues across the House about the potential ‘forever’ nature of the backstop.

“It will introduce a sunset arrangement by which the backstop will fall away.”

The backstop is designed to maintain a soft Irish border if a solution cannot be found through a free trade deal.

But it has proved the most controversial element of the deal, as Brexiteers and the DUP fear it could become permanent.

May’s official spokesman remained tight-lipped on whether the government would whip MPs to back it, stressing that it had not yet been selected by Commons Speaker John Bercow for a vote.

A Labour source said the party will not back the amendment, drastically reducing the chances of it passing.

But it could be a means of demonstrating to the EU how many Tory opponents of the deal could be won over by a concession on an end date.

May on Monday told the Commons the EU has so far been unprepared to agree to an end date to the backstop.

She said: “I recognise that some members wanted to see changes to the withdrawal agreement, a unilateral exit mechanism from the backstop, an end date or rejecting the backstop altogether, although it should be said that that would have risked other EU member states attempting to row back on the significant wins that we have already achieved, such as on control over our waters or on the sovereignty of Gibraltar.

“The simple truth is that the EU was not prepared to agree to this and rejecting the backstop altogether means no deal.

“Whatever version of the future relationship members might want to see—from Norway to Canada, to any number of variations—all require a withdrawal agreement, and any withdrawal agreement would contain the backstop”

Her spokesman stressed afterwards that this reflected her conversations with the EU at December’s European Council summit and afterwards.