I presume he’ll have to go through with it or be labelled as weak, they will definitely be sacked though according to The Garudian:

Three Labour frontbenchers have publicly defied the party leadership and called for a ceasefire in Gaza, hours before Keir Starmer faces one of his biggest rebellions as leader over the issue.

Naz Shah (Bradford West), Afzal Khan (Manchester Gorton) and Helen Hayes (Dulwich and West Norwood) told fellow MPs in the Commons of their intention to vote for an immediate ceasefire, knowing that doing so would result in the loss of their shadow ministerial roles.

Also from their live feed:

Eight shadow ministers quit or face sack from Starmer’s frontbench over Labour’s opposition to Gaza ceasefire

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The rebels on the front bench included Phillips, Rachel Hopkins, Sarah Owen, Afzal Khan, Paula Barker, Naz Shah, Yasmin Quereshi and Andy Slaughter. Two parliamentary private secretaries also left.

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Three Labour frontbenchers resigned in the minutes before the vote, after Starmer made clear that those voting for the ceasefire amendment would be sacked.