THERESA May is facing a defeat in the Commons over Brexit, after Labour signalled they would oppose her Chequers plans.

Shadow foreign secretary Emily Thornberry said her party was preparing to vote against the government when the proposals are put to the vote this autumn.

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The SNP have already said they will not support the government because the plans take the UK out of the single market and customs union, which the party are fundamentally opposed to.

May also faces a rebellion from pro-Brexit Tories who say the proposals would tie the UK too closely to EU rules.

Former foreign secretary Boris Johnson, who quit the Cabinet over the plan, earlier this week described it as wrapping a “suicide vest” around the British constitution while his colleague Steve Baker said it would split the party. Johnson later said the plan would be “substantially worse than the status quo” for businesses.

In an interview yesterday, Thornberry said Labour would not support a vague agreement made with the EU simply because the alternative is leaving the bloc without a deal.

Her comments increase the chances of the Prime Minister being unable to get her deal through the Commons, a development which would throw the Government into chaos.

Labour’s official policy is to judge any agreement May secures in the autumn against six tests they have said it must pass to secure their backing.

However, speaking to the Financial Times, Thornberry said it was unlikely it would do so, and therefore Labour were likely to vote against.

She said the Brexit strategy agreed by the Cabinet at Chequers in July – which seeks to maintain close economic links with the EU – was “nonsense”.

Thornberry added: “Even if they come back in October, November, and they say, ‘this flimsy bit of paper is what you’re going to have to agree to, otherwise there’ll be no deal’ – we’re not going to agree to either of those.”

Thornberry insisted her preferred alternative was a General Election rather than the second referendum supported by many Labour MPs. She said: “I can’t see them coming back with a deal that is going to meet our six tests and I can’t see them coming back with a deal that will unite the Tory Party, for heaven’s sake.

“They are not capable of governing. We’re either going to have a General Election in the autumn or we’re going to have it in the spring.”

Meanwhile, Ruth Davidson yesterday urged May’s critics to “quit the noise” and “let the woman do her job”. The Scottish Conservative leader said May should be given the space to negotiate the best deal possible with the EU.

Davidson called on the rival factions within the Tory Party to recognise the need for compromise over Brexit. She also ruled out leaving Holyrood for Westminster before the next Holyrood election in 2021.