Conservative divisions over Brexit have deepened ahead of a string of key votes as cabinet ministers warned Theresa May to change course.

The prime minister is being pressured on all sides ahead of another momentous week, with speculation mounting that she would call a snap general election if her fourth attempt to pass her Brexit deal fails.

Cabinet minister David Gauke said the UK was facing a “national crisis”. He warned Ms May she could not ignore the will of parliament if MPs unite around a customs union during the second round of “indicative votes” on Monday.

But some 170 Brexiteers, including 10 ministers, were piling the pressure on Ms May over the weekend to leave the EU within weeks – with or without a deal.

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And at least half a dozen Tory MPs were also preparing to row in behind a confirmatory referendum plan. This option secured the backing of the highest number of MPs during the indicative votes on Wednesday.

Downing Street has made it clear that Ms May will bring her Brexit deal back to the Commons for last-ditch attempt, despite her third attempt ending in humiliation on Friday.

But with little hope of winning over core Brexiteers or the Democratic Unionist Party (DUP), Ms May now faces calls to either leave without a deal, call an election or unite MPs behind a much softer exit.

Justice Secretary Mr Gauke said Ms May must consider a plan by veteran Tory Ken Clarke for a customs union – which came closest to securing a majority in the last ballot.

“I think she would need to look very closely at that,” he told the BBC’s Andrew Marr Show.

“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.”

Mr Gauke said Brexit had become a “national crisis” and warned Tory MPs that they may have to accept a “second or third choice” as the party does not have the numbers to push its manifesto pledges through parliament.

Mr Gauke also restated his commitment to resign if the prime minister pursued a no-deal Brexit.

Defence minister Tobias Ellwood also signalled he would support some kind of customs union plan in the votes tomorrow.

He told BBC Radio 4’s The World This Weekend: “Realistically, the Oliver Letwin machine comes back into play on Monday and that will probably tilt towards a customs arrangement or customs union support.”

Mr Ellwood went on: “I agree with supporting the PM but in the absence of the PM’s deal not getting through and if you want to honour the referendum result, then yes I would be supporting something along those lines.”

Five ministers voted for the customs union option last week. Cabinet ministers were ordered to abstain but big-hitters such as Philip Hammond and Amber Rudd are understood to be sympathetic to it.

No 10 hopes that pressure will lead to more Brexiteers reluctantly backing Ms May’s deal after it cut the majority against it from 230 and 149 in the first two votes to 58 on Friday.

But Eurosceptic Tories delivered a show of force, signing a letter organised by Commons leader Andrea Leadsom to demand the UK leaves the EU before the end of May, even if no deal is signed.

A source involved told The Independent: “The letter reaffirmed our commitment to the manifesto and to the PM’s own determination to seek a short extension to Article 50 that avoids the EU elections.”

Elsewhere, efforts to secure a Final Say vote were bolstered by the suggestion another handful of Tories could back it, including former ministers Richard Harrington and Steve Brine, who both resigned last week.

Sources say several Labour MPs who voted against the proposal last week are also saying they are ready to back this new compromise.

Joanna Cherry, the SNP MP behind a revised plan to cancel Brexit if a no-deal scenario seems inevitable, said she had been approached by “many” Labour MPs who want to back it.

MPs will hold a debate on revoking Article 50 on Monday, after the record-breaking petition hit 6 million signatures on Sunday.

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Senior Tories have warned Ms May against calling a snap election, amid fears that the bitterly divided party would be wiped out at the ballot box.

Deputy chair James Cleverly said the party was engaged in “sensible and pragmatic” planning for a snap election but added his voice to a chorus of senior figures pleading with Ms May not to trigger one.

Shadow foreign secretary Emily Thornberry signalled Labour could also try to trigger an election by mounting a fresh vote of confidence in the government – but she could not say when the challenge would take place.