Theresa May is ready to demand the removal of the Irish backstop from her Brexit deal, in a significant toughening of her stance ahead of a fresh Commons showdown.

Downing Street refused, multiple times, to say the prime minister still accepted the withdrawal agreement must include the insurance policy laid down by the EU, to avoid border checks.

Instead, No 10 said the commitment was only to avoiding a hard border by some means – arguing the crushing 230-vote defeat of her deal a fortnight ago meant “changes are going to have to be made”.

The new stance will be seen as encouraging Brexiteer Tories seeking to pass an amendment on Tuesday that “requires the Northern Ireland backstop to be replaced with alternative arrangements”.

However, the EU has repeatedly ruled out reopening the backstop controversy, insisting again today that it “is not open for renegotiation”.

Ms May has previously insisted the backstop is unavoidable in some form, telling MPs in November: “There is no alternative deal that honours our commitments to Northern Ireland which does not involve this insurance policy.”

But, asked if that was still the government’s position, her spokesman replied: “The deal, including the withdrawal agreement and therefore the backstop, was rejected by MPs a by a significant margin.

Brexit deal vote: Opposing groups of protesters gather by parliament Show all 20 1 /20 Brexit deal vote: Opposing groups of protesters gather by parliament Brexit deal vote: Opposing groups of protesters gather by parliament Brexit supporters outside parliament PA Brexit deal vote: Opposing groups of protesters gather by parliament An anti-Brexit protester adjusts her pro-EU wig AFP/Getty Brexit deal vote: Opposing groups of protesters gather by parliament A message to Jeremy Corbyn in support of a people’s vote Getty Brexit deal vote: Opposing groups of protesters gather by parliament A mock Titanic captained by Theresa May heads towards an iceberg in a stunt by campaigning group Avaaz AP Brexit deal vote: Opposing groups of protesters gather by parliament Anti-Brexit protesters outside parliament PA Brexit deal vote: Opposing groups of protesters gather by parliament Protesters of opposing sides are in close contact outside of parliament PA Brexit deal vote: Opposing groups of protesters gather by parliament Paintings of Theresa May and Jeremy Corbyn Getty Brexit deal vote: Opposing groups of protesters gather by parliament Former Ukip leader Nigel Farage speaks to the media at the protests outside parliament Reuters Brexit deal vote: Opposing groups of protesters gather by parliament A pro-Brexit protester in Parliament Square Getty Brexit deal vote: Opposing groups of protesters gather by parliament Opposing protesters share the space outside parliament Getty Brexit deal vote: Opposing groups of protesters gather by parliament An anti-Brexit protester holds EU balloons outside parliament Reuters Brexit deal vote: Opposing groups of protesters gather by parliament Anti-Brexit protesters stand on Westminster Bridge PA Brexit deal vote: Opposing groups of protesters gather by parliament Former Ukip leader Nigel Farage speaks to the media at the protests outside parliament Reuters Brexit deal vote: Opposing groups of protesters gather by parliament Anti-Brexit protesters demonstrate outside the Houses of Parliament EPA Brexit deal vote: Opposing groups of protesters gather by parliament A pro-Brexit protester sets up outside parliament Reuters Brexit deal vote: Opposing groups of protesters gather by parliament An Avaaz campaigner holds a People’s Vote life float Reuters Brexit deal vote: Opposing groups of protesters gather by parliament A demonstrator holds a sign advocating a no-deal Brexit outside parliament AFP/Getty Brexit deal vote: Opposing groups of protesters gather by parliament An anti-Brexit protester waves an EU flag on Westminster Bridge PA Brexit deal vote: Opposing groups of protesters gather by parliament Protesters of opposing sides demonstrate outside parliament AFP/Getty Brexit deal vote: Opposing groups of protesters gather by parliament Protesters of opposing sides stand near parliament Reuters

“So, if we are going to ensure parliamentary approval for the withdrawal agreement, we are going to have make changes to that.”

Asked, again, if the prime minister could now seek a deal without the backstop, the spokesman added: “Our position is that we must avoid a hard border in Northern Ireland and that absolutely remains the case.”

The comments come ahead of a series of crunch votes on Tuesday, including a cross-party attempt to force an extension of the Article 50 deadline, to avoid a no-deal Brexit.

No 10 is expected to back the amendment, put forward by Graham Brady, the Tory backbenchers’ leader, to replace the backstop altogether.

It is seen by some ministers as a way of delivering an ultimatum to the EU, by offering firm proof of the concessions needed in order for the divorce deal to pass.

However, in Brussels, the European Commission ruled out reopening the agreement, saying: “We have a unanimous EU27 position on the withdrawal agreement which reflects the common EU position.

“This withdrawal agreement has been agreed with the UK government, it is endorsed by leaders and is not open for renegotiation.”

Meanwhile, some leading Brexiteers backed away from the Brady amendment, suggesting it will struggle to pass on Tuesday, even if selected.

Iain Duncan Smith told the BBC that Ms May must announce “her intentions”, saying: “We need more than backbench amendments with nods and winks from the government. We need clarity and purpose.”