Labour will consider calling for a vote of no confidence in Theresa May’s government if her Brexit deal is voted down and it appears the UK is at risk of crashing out of the EU without a deal, the party’s Brexit spokesman has said.

Sir Keir Starmer told members of Labour’s parliamentary party on Monday night he was confident parliament would be able to prevent a no-deal scenario, and if necessary the opposition would stage a vote to call for an early election.

“It would be politically unsustainable for any government to deliver a no deal without the consent of parliament,” Starmer told MPs. “There will be opportunities to make the majority against no deal heard. Motions will be tabled, amendments will be pressed and a no-confidence vote can be triggered.”

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A vote of no confidence requires a simple majority of the Commons to pass, although under the Fixed-term Parliaments Act, a 14-day period is allowed for a new government to be formed with the approval of MPs.

Labour believes a majority of MPs would not support a no deal, and that any rejection of May’s final deal by a combination of angry hard Brexiters and opposition parties would lead to chaotic and unstable situation.

In theory, if May’s deal were to be rejected, the UK would head out of the EU on World Trade Organization terms, after MPs voted in principle to quit when the European Union Withdrawal Act was passed.

But the party believes ministers would have to prepare emergency legislation in dozens of areas before the end of March, giving MPs a chance to block an immediate exit. “If Theresa May’s deal fails to command the support of parliament, then we will not stand back and allow her to take this country off a cliff,” Starmer said.

Labour has consistently said it will vote against May’s deal, and if the prime minister cannot get it approved by parliament, that it would attempt to force a general election, although the party would need to win the backing of the DUP, which has been propping up the Conservatives.

But while the Northern Irish party has voiced multiple concerns about May’s deal since it was published, its leaders are suspicious of Labour under its current leadership, because Jeremy Corbyn has said he supports a united Ireland, if it can be brought about under the Good Friday Agreement.

Earlier, Corbyn told the CBI he believed May was cynically using the threat of a no deal to keep her party’s MPs in line. “The choice between Theresa May’s deal and no deal is a false choice, designed to scare people into backing the government.”

The Labour leader said the party would tout its alternative proposal in which the UK would remain in the customs union and retain a close relationship with the single market, in an attempt to gain some support in the business community.

“The government is trying to force through this bad deal by threatening us all with the chaos and serious damage to our economy of a no-deal outcome. But the prime minister knows that no deal isn’t a real option,” Corbyn said.