His stark warning came after members of his frontbench revolted over his stance, and as protesters mobbed his car as he attempted to leave after his speech at Chatham House – just weeks after he a campaigner stormed the stage and doused him in glitter at the Labour conference.
Sir Keir has called for short “humanitarian” pauses in the fighting, as distinct from a ceasefire, in a bid to allow aid into Gaza and hostages and foreign nationals out.
But as the crisis facing Labour over Israel’s war on Hamas grows, several frontbenchers have defied the principle of “collective responsibility” to break the party line and call for a ceasefire.
Frontbencher Alex Cunningham called for an “immediate” halt to the fighting less than an hour before Sir Keir delivered his speech, and MPs have warned the party is losing voters over its position.
Sir Keir said he understood the calls for a ceasefire, but it was “not the call that we should be making as things stand”.
“Hamas would be emboldened and start preparing for future violence immediately,” if there was a ceasefire, he said.
The Labour leader insisted he took collective responsibility seriously, but gave no indication he planned to sack those who had spoken out.
He also insisted there was “unity” in Labour over the “key issues” including a two-state solution to the conflict.
But a humanitarian pause is the “only credible approach”, which could see “the urgent alleviation of Palestinian suffering”.
While he defended Israel’s right to defend itself, he warned it could not block supplies of fuel to Gaza, amid concerns it would be used by Hamas. He also said there was a “rising temperature on British streets” as a result of the conflict.
Rishi Sunak and US President Joe Biden have also called for temporary humanitarian pauses to allow aid into Gaza – and hostages and British nationals out. But Sir Keir is facing mounting difficulties keeping discipline within his ranks.
On Monday Labour suspended the whip from senior MP Andy McDonald for using the phrase “between the river and the sea” in a pro-Palestine rally speech.
Scottish Labour leader Anas Sarwar also said that Sir Keir had “hurt” Muslim communities by suggesting Israel could withhold water and energy from the people of Gaza. Sir Keir later apologised for that statement saying he thought he was answering a question on Israel’s right to defend itself.
In an interview with the Daily Record, the Scottish Labour leader said: “It was hurtful and I think he would accept it was hurtful. He accepts that it is not his position, and it never was his position.”
Humanitarian pauses typically last for short periods of time with the aim of providing aid and support rather than achieving long-term political solutions, according to the United Nations.
Ceasefires are intended to be long-term and usually seek to allow parties to engage in talks, including the possibility of reaching a permanent political settlement.
Labour’s Sir Chris Bryant said a ceasefire would not work because the Israel-Hamas conflict was not “some kind of football match”.
The shadow creative minister said there are not “two sides”, with Hamas not having committed to “laying down their weapons or returning the hostages being kept against their will”.
Shadow ministers Yasmin Qureshi, Jess Phillips, and Imran Hussain are among the figures who have joined calls for an end to the fighting.
Labour has also been at odds over its stance on Israel with devolved mayors like Andy Burnham and Sadiq Khan, and with Labour-led councils across England.
But the party is not likely to sack its internal critics from frontbench roles, and will instead “continue engaging” with them, shadow science secretary Peter Kyle said on Sunday.
Middlesbrough MP Mr McDonald was suspended by Labour after what a party spokesperson said were “deeply offensive” remarks made at a speech during a pro-Palestine rally.
Mr McDonald said his reference to the phrase “between the river and the sea” was part of a “heartfelt plea” for peace in the region.
A slogan used by pro-Palestinian demonstrators, “from the river to the sea, Palestine will be free,” has been described as antisemitic by critics, with home secretary Suella Braverman claiming that it is “widely understood” to call for the destruction of Israel.
Sir Chris said the phrase was “antagonistic” and “suggests that Israel should no longer exist”.
Mr McDonald, now sitting as an independent, said he would fully cooperate with the investigation into his suspension and trusted “that the whip will be restored”.
Mr Sunak on Monday sacked a senior Tory from his job as a ministerial aide for calling breaking ranks and calling for a ceasefire.
It allows the Tories to make a sharp contrast with the row within Labour ranks, with at least 13 frontbenchers openly challenging Sir Keir’s refusal to back a ceasefire without any disciplinary action.