Imran Hussain, the MP for Bradford East, announced that he was quitting his role as shadow minister for work to be able to “strongly advocate” for a ceasefire.
Sir Keir is now fighting to maintain discipline, with more than 65 MPs, including 18 frontbenchers, backing a ceasefire. Almost 50 Labour councillors have quit and hundreds of local representatives oppose the leadership’s stance.
The resignation comes as left-wing Labour MP Zarah Sultana has tabled an amendment to the King’s Speech calling for an “immediate ceasefire”.
The amendment is backed by MPs from six parties and could be voted on next Wednesday at the conclusion of the King’s Speech debate. Some 14 Labour left-wingers, including John McDonnell, Nadia Whittome, Rosena Allin-Khan and Richard Burgon, have backed the move.
A Labour MP told The Independent: “I don’t think Imran Hussain will be the last. MPs will go back to their constituents – who will point to Imran and say ‘Why can’t you do that?’ So I imagine it could tip a few more into resigning.”
They added: “There’s still a lot of anger towards Starmer. The s***show isn’t going to go away. A vote in the Commons would be a moment of truth for MP to show their constituents they support for a ceasefire.”
One Labour frontbencher told The Independent Mr Hussain’s resignation could put pressure on others in the party who have spoken out on the ceasefire.
They said: “It was always a surprise Imran came back on the front bench as he was a big Jeremy [Corbyn] fan. It will put a bit of pressure on a few colleagues, but after four weeks I don’t think it will be a game changer.”
In his letter to Sir Keir, Mr Hussain said he was “deeply troubled” by the Labour leader’s interview on LBC in which he appeared to suggest that the Israeli government had a right to withhold water and power from citizens in Gaza.
The MP, a member of the Socialist Campaign Group, said he had been “proud” to work alongside Sir Keir and his deputy Angela Rayner, but could not “in all good conscience” push for a ceasefire while remaining on the frontbench.
Sir Keir’s shadow education secretary Bridget Phillipson said that Mr Hussain had “reached his own decision” and maintained that the party leadership did not see a ceasefire as the right move.
In a warning to others, Ms Phillipson told Times Radio: “I would urge colleagues to always carefully choose their words where it comes to fast-moving international events, and where it isn’t always clear or apparent very quickly what is happening on the ground.”
Despite support for a ceasefire among shadow ministers, Mr Hussain was the only Labour frontbencher to have backed a fresh parliamentary motion calling for a ceasefire tabled by Mr Burgon on Tuesday.
Sir Keir’s team has made clear to frontbenchers that signing the latest motion – refreshed after the return of parliament – would not be compatible with them staying in their jobs.
One Labour shadow minister, who supports Sir Keir’s stance, said the party has “managed to come together a lot more in the past week”, adding: “I think the vast majority believe he’s in the right place.” Another frontbencher told The Independent that they “doubt it will spread beyond [Mr Hussain]”.
However, alongside Ms Zultana’s amendment, Labour left-wingers are looking at other options to force the ceasefire issue in parliament – including work with the SNP on an opposition day debate, which would end with a non-binding vote.
Labour frontbencher Naz Shah – a shadow Home Office minister who has backed a ceasefire – wiped at her eyes with a tissue in the Commons after telling MPs of the plight of children in Gaza and calling on the UK to “ramp up its effort to end the bloodshed”.
A Labour Party spokesman said: “Labour fully understands calls for a ceasefire ... But a ceasefire now will only freeze this conflict and would leave hostages in Gaza and Hamas with the infrastructure and capability to carry out the sort of attack we saw on October 7.”
Ms Sultana, Labour MP for Coventry South, told the Commons it was time to “finally do what is right and demand an immediate ceasefire to end the bloodshed”. She added: “More than 10,000 Palestinians have been slaughtered in Israel’s assault, nearly half of whom are children.”
Paul Bristow, the Tory MP for Peterborough recently sacked as an aide, spoke about a constituent from Gaza who has lost contact with his family. “How much longer do we have to wait until this suffering ends and humanitarian aid can reach people like my constituent’s family?”
Labour’s shadow Foreign Office minister Lisa Nandy said on Wednesday that the siege conditions imposed on Gaza were “totally unacceptable” – but said humanitarian pauses were the “only viable prospect”.
Speaking in the Commons, Ms Nandy said: “For weeks the international community has demanded the siege conditions on Gaza be lifted, but it still has not happened – this is totally unacceptable and it cannot continue.”
It follows strong remarks by Labour’s shadow foreign secretary David Lammy accused Israeli ministers of “unacceptable and offensive rhetoric” about Palestinians and condemned acts of “violence and extremism” by Israeli settlers in the West Bank.
It comes as Momentum founder Jon Landsman said he found it “difficult to relate to how much of the left responded” to Hamas’ attack on Israel.
Mr Landsman told the New Statesman that the “from the river to the sea” chant heard at pro-Palestine marches was wrong, and said Mr Corbyn “had a real problem talking to Jewish communal bodies”.