A House of Commons debate over competing calls for a Gaza ceasefire degenerated into a furious row over parliamentary procedure, after the Speaker Sir Lindsay Hoyle defied custom and precedent by deciding to allow a vote on a Labour amendment to a motion introduced by the Scottish National Party.

Amid evident rancour, as the debate drew to a close the Leader of the House Penny Mordaunt announced that the Conservatives would be playing no further part in the proceedings, and dozens of both Tory and SNP members staged a walk-out.

The outcome was that the Labour version became certain to pass. It duly was, without a formal division.

Like the motion tabled by the SNP, this demanded “an immediate humanitarian ceasefire”. But it added condemnation of “the terrorism of Hamas” and stated that Israel “cannot be expected to cease fighting if Hamas continues with violence”. It also excised wording in the SNP motion condemning Israel’s “collective punishment” of Palestinians – which would constitute a war crime.

Shadow foreign secretary David Lammy told the House that the SNP motion was “one sided”, saying “you could not ask Israel to lay down its arms without also asking Hamas”.

The procedural row blew up because the debate was scheduled on one of the 20 “opposition days” in the annual parliamentary calendar. Convention decrees that one opposition party – in this case, Labour – would normally be unable to secure a vote to amend a motion tabled by another, which today meant the SNP.

If Hoyle had followed precedent and refused to allow a vote on the Labour amendment, Labour MPs would have been faced with a simple choice: either backing the SNP motion, complete with its “collective punishment” claim and absence of criticism of Hamas, or voting with the Tories against it.

It was widely thought that this would have led to a rebellion against party leader Sir Keir Starmer significantly bigger than the revolt he faced in November, when eight shadow ministers resigned their posts because they wanted to support a ceasefire then, at a time when the leadership was rejecting it.

The bitterness was further stoked by claims – which were hotly denied – that Labour had threatened not to support Hoyle’s re-election as Speaker after the general election later this year unless he allowed a vote on the Labour amendment.

Having made his decision, Hoyle left the Speaker’s chair leaving his deputy Rosie Winterton to chair the debate. He returned after the vote to apologise and said he would meet with party leaders to discuss the day’s events in the coming days.

The debate saw numerous passionate contributions from all sides.

Some MPs dwelt on the suffering of both Palestinian civilians and the Israeli hostages still being held by Hamas. One was Liberal Democrat foreign affairs spokeswoman Layla Moran, who is partly of Palestinian descent, who asked the House: “Can we please try and send a message to the Palestinian people perishing in their tens of thousands on the ground, and to those hostage families, that fundamentally we need this to stop now?”

Other – mainly from the SNP and the Labour left – focused on the Palestinians. One among many was Labour’s Rosena Allin-Khan, who spoke of “children under five talking of wanting to take their own lives because they have watched their siblings hanging from buildings, dead; they have watched their parents exsanguinating in front of them, and are now left alone to face this world”.

She added: “Yes we need a peace process, yes the hostages must be freed, yes the wheels of international humanitarian law must turn, and yes, the Palestinian people must have a recognised state, but first, today, this minute, now, we must have an immediate ceasefire to save tens of thousands of lives.”

The former immigration minister Robert Jenrick pointed out that there was a “very real danger that our emerging position, and certainly that of other parties, will leave Hamas’ terrorist organisation partially intact. That is an intolerable situation for Israel, it sends a very clear message that using human shields works, and we will not allow Israel to fully defend itself.”

Sir Michael Ellis, the Tory former attorney-general, added that calling for a ceasefire would help Hamas, saying: “Israel has been through multiple rounds of conflict, initiated by the genocidal Hamas terror groups in Gaza and this motion – should it achieve its objective – would cement the prospects of many more such incursions or attacks in the future, and that is of course exactly what Hamas wants, to secure endless opportunities to destroy Israel granted by the confused logic of that motion.”

As the day drew to a close, it seemed clear that despite the chaos, the House’s centre of gravity had shifted further away from support for continuing Israeli military action, with pressure on its government from UK MPs likely only to increase.