Tom Watson piles pressure on Jeremy Corbyn after backing British air strikes on Isis in Syria

Four senior Labour MPs have broken cover to claim that Jeremy Corbyn’s leadership of the party was “unsustainable”, as his own deputy became the latest shadow cabinet minister to defy him on British military action in Syria.

Tom Watson piled pressure on Mr Corbyn by publicly supporting the shadow Foreign Secretary Hilary Benn, saying that the threat of Isis meant Britain should participate in air strikes. Asked if that made his position untenable, he replied pointedly: “I am the deputy leader of the Labour Party with a mandate.”

Meanwhile, four MPs took to the airwaves to question Mr Corbyn’s suitability as party leader. Paul Flynn said that Labour was in a “terrible, terrible mess”, while Frank Field, one of just 36 MPs to nominate Mr Corbyn, said that the party needed “an alternative leader”.

They were backed by the former Labour Home Office minister Fiona Mactaggart, who said his “weak leadership” was causing damaging divisions and failing the party’s responsibility to hold the Government to account.

“He hasn’t got a strategy to lead the party from where it is to where it needs to be and the people of the country can see that,” she said. “I think it [his leadership] is probably unsustainable.” The former defence minister John Spellar added that if “anyone should resign” over Syria it should be Mr Corbyn.

“How does Jeremy Corbyn and his tiny band of Trots in the bunker think they’ve got the unique view on it all?”

Some of Mr Corbyn’s team still hope they can persuade enough Labour MPs to back their position opposing air strikes over the weekend before a second meeting of the Shadow Cabinet on 30 November.

But others believe he will have no choice but to offer Labour MPs a free vote on the issue when a vote is called by the Government.

“This will end up as a free vote,” said one, while another said they thought the mood was turning against David Cameron and in particular his claim that British air strikes would be backed up by 70,000 Syrian opposition fighters.

“We are not in the position of conceding a free vote yet,” they said. “Let’s see what happens over the weekend when MPs go back to their constituencies.”

They added: “Any decision on a whip will be Jeremy’s as leader rather than the Shadow Cabinet. The first principle is that Jeremy appointed the Shadow Cabinet and they serve under him.”

The issue may not be resolved until a meeting of the full Parliamentary Labour Party late on 30 November which Mr Corbyn is due to address.

The shadow Chancellor, John McDonnell, one of Mr Corbyn’s closest allies, appealed for calm, insisting the party was “working through the issues”.

“Don’t mistake democracy for division,” he tweeted.

However, Mr Corbyn’s position has not been strengthened by an appeal by the Socialist President of France, François Hollande, for British MPs to back Mr Cameron.

“We want to destroy terrorism … I hope that the House of Commons will be able to meet the request of Prime Minister Cameron,” he said.

Mr Corbyn also faced criticism from two senior MPs after one of his closest aides was allowed back into Labour ranks despite calling for voters to support a rival party at the election.

The decision by the party’s ruling National Executive Committee to readmit Andrew Fisher, a senior policy adviser to Mr Corbyn, with a warning, was condemned by the former minister Caroline Flint and the Mitcham and Morden MP Siobhain McDonagh. They complained that “enormous pressure” had been exerted to avoid Mr Fisher’s expulsion and protested that there seemed to be “one rule for members and one rule for those who work for the party leader”.

Mr Fisher had posted a message on Twitter urging people to back the Class War candidate in Croydon South instead of Labour’s Emily Benn. He apologised “unreservedly” for the tweet and promised there would be no repeat of such messages as the NEC mounted an investigation.

A Labour spokeswoman said: “Andrew Fisher has been issued with an NEC warning and his suspension has been lifted with immediate effect.”

There was some good news for Mr Corbyn when the Fire Brigades Union, which has about 38,000 members, voted at a special conference to reaffiliate to Labour after more than a decade. It split from the party following a pay dispute with the Blair government.

Matt Wrack, its general secretary, said: “Firefighters recognise that the Labour Party has changed for the better since the election of Jeremy Corbyn, who has given our members and supporters hope that we can shift the political debate in favour of working people.”

The Labour leader said: “Their vote marks a milestone in the building of our new politics and our Labour movement.”

Q | Which way will Labour vote on British military intervention in Syria?

A | After days of very public rows it now looks fairly certain that Labour MPs will be given a free vote on whether to back David Cameron’s call for air strikes against Isis in Syria. Despite being passionately opposed to military action, Jeremy Corbyn has accepted that he does not have the support of his Shadow Cabinet to enforce his view on parliamentary colleagues.

Q | But couldn’t he just impose a whip on MPs and dare them to defy him?

A | Not really. Technically the decision to impose the whip lies with the Shadow Cabinet and after a meeting on Thursday it was clear that around two-thirds of his team were opposed to his position. Mr Corbyn would be on very shaky constitutional ground if he tried to impose a whip unilaterally – not to mention the damage it would do to his credibility.

Q | Does that mean a majority of Labour MPs will now vote for air strikes?

A | Not necessarily. Mr Corbyn will use a meeting of the Parliamentary Labour Party on Monday night to appeal to MPs to back his position – even though the vote on Syria will not be whipped. Significant numbers of Labour MPs have serious reservations about Mr Cameron’s plan. Yesterday Yvette Cooper, the former shadow Home Secretary, called for MPs to be given intelligence briefings from the Foreign Office ahead of any vote. This is unlikely to happen – but emphasises the nervousness of many centrist Labour MPs over what to do. A lot of Labour MPs may now end up abstaining.

Q | What does all this mean for Mr Corbyn’s leadership?

A | By any measure this has not been a good week for the Labour leader – but it doesn’t necessarily increase the probability of a challenge to his leadership. Labour MPs know the party membership is more inclined to Mr Corbyn’s view on military action than they are. Their conundrum is they also know the wider electorate (who put them in their jobs) don’t agree.

Oliver Wright

Simon Fletcher: A veteran left-wing activist in London, he worked for Ken Livingstone for 12 years, including eight as the former Mayor’s chief of staff. He ran Mr Corbyn’s leadership campaign and is now his chief of staff.

Katy Clark: A former MP who was a stalwart of the Campaign group. She lost her seat in the SNP landslide in May, but Mr Corbyn brought her back to Westminster as his political secretary.

Neale Coleman: A former Westminster councillor who played a prominent role in uncovering the “homes for votes” scandal, he was a policy adviser to both Ken Livingstone and Boris Johnson. He is Labour’s executive director of policy and rebuttal.

Andrew Fisher: His recruitment as a senior policy adviser to Mr Corbyn was among the Labour leader’s most controversial appointments. An economist and the former head of policy at the PCS public service union, he has a record of attacking party frontbenchers online.

Seumas Milne: As Labour’s executive director of communications and strategy, he has the task of handling Mr Corbyn’s dealings with the media. He is “on leave” from The Guardian, where he was a columnist and associate editor – and a rare voice in the media championing the Islington MP’s leadership bid.

John McDonnell: The shadow Chancellor is Mr Corbyn’s closest ally in Parliament. He has been immersed in left-wing politics for nearly 40 years including a spell as chairman of finance at the Greater London Council.

Nigel Morris