The Labour leader had ordered the party's MPs to abstain in a vote on a motion tabled by the SNP on Wednesday.
But 10 frontbenchers including Jess Phillips, Afzal Khan and Yasmin Qureshi left their roles to back the motion,which was defeated by 125 votes to 294.
Find out how your MP voted using the search box below.
Click hereif you cannot see the look-up. Data from Commons Votes Services.
The motion, one of several proposed amendments to the King's Speech, condemned Hamas's "horrific" killings and hostage-taking, but urged the government to press all sides for "an immediate ceasefire".
Labour's official position, like that of the government, is to call for "humanitarian pauses" in the fighting to allow more aid through. Sir Keir set this out in a separate amendment which was also defeated, but was backed by 160 Labour MPs.
Eight shadow ministers either resigned or were sacked after voting for the ceasefire motion. They were:
Two others, Dan Carden and Mary Foy, left their posts as parliamentary aides.
Produced by Maryam Ahmed, Steven Connor, Rob England, Christine Jeavans, Wesley Stephenson, John Walton
Sir Keir Starmer has suffered a major rebellion over his stance on the Israel-Gaza war, with 56 Labour MPs voting for an immediate ceasefire.