Jeremy Corbyn has walked out of an early evening meeting of party leaders with Theresa May after he realised that the prime minister had invited the Independent Group spokesman, Chuka Umunna.

The Labour leader had been due to meet May to discuss the Brexit crisis alongside the SNP’s Ian Blackford, the Lib Dems’ Vince Cable, and the parliamentary leaders of Plaid Cymru and the Greens.

But those present said he quit the meeting once he realised that former Labour MP Umunna, who is not a party leader but the spokesman for the newly formed group of MPs made up of Tory and Labour defectors, had also been invited.

A Labour spokesperson said afterwards: “It was not the meeting that had been agreed … the terms were broken” and that the party was talking to No 10 about holding a face-to-face meeting that Corbyn had earlier proposed at prime minister’s questions.

Umunna, however, claimed it was “extraordinary behaviour” and added: “I don’t think this is what people expect from a leader of the opposition at a time of crisis when the people that elect us to come together and see if there is a way forward.”

Cable said: “Jeremy Corbyn’s kinder, gentler politics was found wanting as he stomped out of the meeting before it began rather than breathe the same air as Chuka Umunna.”

Corbyn subsequently spoke to May on the telephone, and complained that she had offered nothing new beyond a choice between her deal and no deal next week. The Labour leader said the prime minister was “in complete denial about the scale of the crisis we are facing and unable to offer the leadership the country needs”.

During the day, there were also signs that a small number of Labour MPs were now prepared to switch sides and back May’s Brexit deal after the EU said it had to pass the Commons next week.

Lisa Nandy, the Labour MP for Wigan, said she would be “minded to vote” for May’s Brexit deal if the prime minister accepted an amendment submitted in her name and that of party colleague Gareth Snell on Wednesday evening.

Earlier in the day, Snell said he too was considering a switch to backing May. “I’m increasingly coming to the conclusion that the only way to stop no deal may be to vote for her deal. I’m not happy about that. I think that was her plan all along.”

Nandy, however, would not predict exactly how many Labour MPs might support her amendment, other than to say that she thought it would win the backing of “a number of us”. She added: “It is the best prospect May has.”

But late on Wednesday, after Nandy had said she might back the prime minister, the MP reacted badly to May’s evening televised statement where she expressed frustration with parliament repeatedly blocking her Brexit deal.

“Pitting parliament against the people in the current environment is dangerous and reckless,” Nandy said. “Now she’s attacking the MPs whose votes she needs. It will have cost her support.”

The prime minister would need the support of about 30 Labour MPs to have a chance of forcing her deal through, assuming she managed to also sign up the Democratic Unionist party, to offset the estimated two dozen Tory diehards who will never support the prime minister.

Other Labour MPs reacted with a mixture of anger and scepticism to Nandy and Snell’s intervention. One said that he and party colleagues were very unhappy with the idea that the pair were willing to engage in discussions with the Tories.

The amendment, if accepted, would require May – or whoever is the prime minister – to accept three conditions to the next phase of Brexit negotiations on a future trade deal that would begin if the withdrawal agreement is passed by the Commons. It would require the prime minister to agree a negotiating mandate with the Commons before trade deal talks start, report back on progress every three months, and require a confirmatory vote by MPs at the end of the process.

Nandy said she had discussed the idea with May “three or four times” and said that she would link the amendment to a third meaningful vote next week. Otherwise it would be put forward on Monday, when MPs are due to discuss the next Brexit steps.

Earlier in the day, Sir Keir Starmer, the shadow Brexit secretary, secured an emergency Commons debate to discuss the crisis by invoking a procedure that required the support of at least 40 MPs.

But a cross-party attempt to submit an amendment to that debate, calling for a long delay to Brexit, collapsed after it emerged that Labour’s leadership was not prepared to support a lengthy extension, sources said.