A confidence vote in Theresa May’s Government is “inevitable” next week if the Prime Minister fails to offer decisive changes to her beleaguered Brexit Plan, SNP sources have suggested.

Mrs May is due to address MPs on Monday in a Commons statement but the expectation at Westminster is that she will simply say that her efforts to get “further assurances” on the Irish backstop – the guarantee of an open land border on the island of Ireland should a UK-EU trade deal not be agreed by December 2020 – are continuing.

On Tuesday, Ian Blackford, the SNP leader at Westminster, urged Jeremy Corbyn to “take responsibility” and show leadership by calling a confidence vote in the UK Government and suggested that if he did not do so by the “close of business tonight,” then he, as the leader of the third party, would do so. However, nothing happened.

The SNP pressure on the Labour leader led to an inter-party spat; Mr Corbyn cancelled a meeting with Mr Blackford.

John McDonnell, the Shadow Chancellor, claimed Nicola Sturgeon was “terrified” of a general election because Labour was “breathing down their necks in Scotland; we'll take seats off them in so many marginals".

The SNP strongly denied this, claimed the Shadow Chancellor was “clutching at straws” and insisted it would relish a fresh election as polls suggested, in fact, it would take seats from Labour in Scotland.

Nationalist sources defended their decision not to push the button on a censure motion, saying events were moving so fast that the party had to be certain it could maximise the chances of success.

But one senior party insider suggested that a confidence vote next week was now “inevitable”. Another, when asked if he agreed, said: “I think so.”

In the Commons, Pete Wishart, the SNP’s Shadow Commons Leader, noting how the Tory Government was “there for the taking; divided, wounded and unable to govern,” said he had no idea what the Labour Party was waiting for.

“We could be shot of them pretty soon,” declared the Perth MP. “So if Labour is not going to put in a motion of no confidence in this Government, it will be left to the other opposition parties of this House to do so,” he added.

Andrea Leadsom, the Tory frontbencher, hit back, saying: “If the official Opposition dispute that the Government have the confidence of the House, it is for them to test it via a motion under the terms of the Fixed-term Parliaments Act 2011.”

Earlier this week, a spokesman for Mr Corbyn suggested the party would indeed put down a confidence motion, noting it was a question of “not if but when”.

Meanwhile, the Prime Minister was at an EU summit, pleading with her fellow leaders to enable the UK to determine when it could leave the backstop as part of the Brexit deal.

While several EU figures have suggested that “clarifications” could be made to help explain the temporary nature of any backstop, it is thought this would be nowhere near enough to satisfy the qualms of Tory Brexiteers and the Democratic Unionists.

The PM said there was no prospect of an “immediate breakthrough” while No 10 confirmed that the rescheduling of the vote on the deferred Brexit deal would not happen before Christmas but "as soon as possible" in January ie before the 21st.

Sebastian Kurz, the Austrian Chancellor, suggested the EU could call an emergency summit in January to agree “additional assurances,” which could be attached to the political declaration on future relations addressing the concerns of MPs.

But Tory backbencher Sam Gyimah, who resigned as the Science Minister over the PM’s Brexit Plan, argued it would “take a Christmas miracle” for her to come back from Brussels with something that would get through Parliament.

Elsewhere, Tobias Ellwood, the Defence Minister, made clear a no-deal scenario was “not an option”.