THERESA May has been warned by one of her senior ministers that it would not be sustainable to ignore MPs if they vote for a softer Brexit today, ahead of another crucial week in Westminster.

Justice Secretary David Gauke said the Prime Minister will need to “look very closely” at the option if it receives majority support in the Commons.

It comes as MPs prepare for another round of indicative votes on alternative Brexit options. Last week, all eight alternatives failed to find a majority, but staying in a customs union with the EU and holding a second referendum were only narrowly rejected.

Speaking on the BBC’s Andrew Marr Show, Mr Gauke said: “If Parliament is voting overwhelmingly against leaving the European Union without a deal but is voting in favour of a softer Brexit, then I don’t think it’s sustainable to ignore Parliament’s position and therefore leave without a deal.”

He added: “I think we also have to recognise my party does not have the votes to get its manifesto position through the House of Commons at the moment.

“We are in an environment when it is not just about going for your first choice. Sometimes you do have to accept your second or third choice in order to avoid an outcome you consider to be even worse.”

Labour’s deputy leader Tom Watson said holding a second referendum was the only way to bring the country back together, adding: “A People’s Vote is the solution, not an option.”

He insisted it was “inconceivable” a public vote would not be included in the next Labour manifesto if a general election is called.

After 170 Tory MPs – including 10 members of the Cabinet – wrote to Mrs May urging her to take the UK out of the EU as quickly as possible, Mr Gauke insisted he would resign if the Government pushed for a no-deal exit.

Despite MPs rejecting Mrs May’s deal for a third time on Friday, Downing Street has made clear she intends to bring it back to the Commons for a fourth vote, possibly tomorrow or Wednesday.

Mr Gauke said that while he believed it still remained the best option, the Government had to accept that – so far – it did not have the numbers to get it through Parliament.

He said that if MPs did coalesce around a plan by veteran Tory Ken Clarke for a customs union – the option which came closest to securing a majority in the last round of votes – ministers should be prepared to consider it.

Mrs May has so far strongly rejected the idea of a customs union, insisting it goes against the Tory general election manifesto and would prevent Britain striking trade deals around the world. Such a move would also infuriate Brexiters and inevitably lead to ministerial resignations.

It came as Tory MPs warned Mrs May they will resist any attempt to call a snap election in a bid to end the Brexit crisis engulfing the party – with leading figures on both the pro-Brexit and pro-EU wings fearing they would face electoral disaster.

With the Government now up against a new EU deadline of April 12, former Tory prime minister Sir John Major insisted a time-limited, cross-party “unity government” may be required to break the deadlock.

He warned a no-deal Brexit “would put at risk the union with Scotland, and over time the union with Northern Ireland”.

Speaking to the BBC, he added: “That is too high a price, and I think that’s something Parliament has to bear in mind.”

Meanwhile, SNP Westminster leader Ian Blackford urged colleagues from across the political divide to back motions to revoke Article 50 – halting the Brexit process – or hold a second EU referendum.

He said MPs must use today’s indicative votes to keep remaining in the EU firmly on the table and rule out the worst forms of Brexit – including any that involve leaving the single market and ending freedom of movement.

SNP MPs are currently considering whether to back the “Common Market 2.0” proposal spearheaded by Tory backbencher Nick Boles. This would see the UK staying in the single market and a form of customs union.

However, SNP MP Pete Wishart told The Herald the party wanted to be “absolutely certain” this was advocating full single market membership and freedom of movement. He said there were also concerns over the temporary nature of the proposed customs arrangements, but added: “We remain open to it.”

SNP MP Joanna Cherry is also putting forward a revised plan to revoke Article 50 if a no-deal scenario seems inevitable.

This was defeated by 109 votes on Wednesday, but Ms Cherry said she had since been working closely with some Labour politicians to find a more “palatable” way of wording the proposal.

Scottish Labour MP Paul Sweeney has suggested that he and others are “sympathetic” to the idea, which he described as a “sensible safeguard”.

Elsewhere, shadow foreign secretary Emily Thornberry launched a highly personal attack on Mrs May, insisting her judgment was impaired and she was “out of control”.

She told Sky News’s Sophy Ridge On Sunday programme: “She is not listening to anyone. No one knows what it is that she is going to do next. I think her judgment has been undermined.”

It came as a petition calling on the Government to halt Brexit passed six million signatures.

The Revoke Article 50 petition, due to be debated by MPs today, is the best-supported proposal in the history of the House of Commons e-petitions website.

Rejecting the claim that EU withdrawal is the “will of the people”, it calls for the revocation of the Article 50 letter informing the European Council of the UK’s intention to leave.