Grassroot Labour groups are gearing up for a conference showdown with Andy Burnham over Gaza, as the party’s members seek to force the prime minister to recognise Israel’s actions as a genocide, and impose a full arms embargo.

Momentum, Labour & Palestine, the Campaign for Labour Party Democracy and Palestine Solidarity Campaign (PSC) will ramp up pressure on Burnham before his first Labour conference as prime minister next month.

The groups are circulating a model motion for Labour members to submit to conference, demanding the government accepts the findings of a UN commission of inquiry that Israel has committed genocide in Gaza, and to impose “comprehensive sanctions”, including a full arms embargo.

The motion also demands that ministers impose comprehensive sanctions and ban trade that “aids or assists” Israel’s military actions in Gaza or its unlawful presence in the occupied Palestinian territories, including trade with Israeli settlements.

Burnham’s refusal to use the term genocide has caused frustration among many Labour MPs, not just those on the left of the party. Several MPs told the Guardian they welcomed Burnham’s apology, but wanted him to go further and explicitly “acknowledge Israel’s actions in Gaza as a genocide”.

The prime minister has apologised for the party’s handling of Gaza under Starmer’s leadership, saying Labour had “not got it right”.

Weeks before he became prime minister, Burnham told the Guardian: “We’ve got to do more to put pressure on the Israeli government … let’s be honest, the UK was too slow to call for a ceasefire. And we must now do more to strengthen our approach.” But he stopped short of describing what was happening on the ground in Gaza as a genocide, saying that while there was “increasing evidence” of war crimes, it was for the international courts to make that ruling.

The UK government has signalled that further measures against Israel could be coming. Ed Miliband, the foreign secretary, said ministers would set out a “comprehensive set of measures” after Israel moved ahead with its controversial E1 settlement plans in the occupied West Bank.

In the run-up to the 2025 party conference, Labour sought to rule out motions on Palestine being debated at the conference, which triggered accusations that the party was trying to stifle debate. About 30 motions about Palestine from local parties were struck off last September.

But conference went against the then leadership and backed an emergency motion proposed by Unite, Labour’s biggest union, that the Labour government should “employ all means reasonably available to it to prevent the commission of genocide in Gaza” and “fully suspend the arms trade with Israel and the UK-Israel trade and partnership agreement”.

The Labour groups are seeking to force the issue back on the conference floor this year, presenting it as an early test of Burnham’s promise not to “stifle debate” within Labour. Momentum is launching a petition aimed at current and former Labour members and voters, urging Burnham to stick to the position backed by delegates last year.

Burnham is facing growing pressure beyond Labour’s left after more than 100 British and French diplomats wrote to Burnham and Emmanuel Macron urging tough action against Israel, including suspending arms transfers and taking further trade measures. Emily Thornberry this week said Britain should ban imports from the occupied West Bank amid growing concern over Israeli settlement expansion.

It comes as Labour is hoping to win back progressive voters who deserted the party under Starmer. Polling commissioned by PSC and Friends of the Earth this summer found 67% of voters who switched from Labour to the Greens cited Palestine as a factor in their decision to leave Labour.

Sasha Das Gupta, co-chair of Momentum, said: “Burnham must not allow the same failures that alienated voters under Starmer to continue. He must listen to the majority of the labour movement and take decisive action on Gaza.”

Dan Iley-Williamson, political organiser at the Palestine Solidarity Campaign, said: “Burnham’s apology for Labour’s previous stance on Palestine will mean nothing if his government doesn’t decisively break with Keir Starmer’s complicity in Israel’s crimes.

“Burnham could use this Labour conference to show the country he will make this break with Starmer. If he doesn’t, he risks the same fate: plummeting support and a legacy of infamy.”

The UK Foreign Office was approached for comment.