London's mayor Sir Sadiq Khan has for the first time described the situation in Gaza as a "genocide", becoming the most senior Labour figure to contradict the government's official position.
Earlier this week, a UN Commission said a genocide was taking place in Gaza - something repeatedly denied by Israel.
Meanwhile, Sir Keir Starmer has been under pressure to raise Israel's bombardment of the territory with Donald Trump during his state visit to the UK.
The prime minister is due to have discussions with the president today, but reports suggest he will delay formally recognising a Palestinian state until this weekend, after Mr Trump has left Britain.
It is claimed the government wants to avoid the issue dominating a news conference the two men plan to hold, according to The Times.
The prime minister has found himself at odds with the US administration over the move, which is opposed to official recognition of Palestine.
The mayor of London, who has engaged in a long-running spat with Mr Trump, has added to the political tension by contradicting official Labour policy at a people's question time event on Wednesday.
"I think it's inescapable to draw the conclusion in Gaza we are seeing before our very eyes a genocide," said Sir Sadiq.
Sir Keir has previously pledged to recognise Palestinian statehood ahead of next week's United Nations General Assembly in New York if Israel does not meet a series of conditions to improve the humanitarian situation in Gaza.
Other nations, including France, Australia and Canada, have said they plan to take the same step at the UN gathering.
Explainer: What does recognising a Palestinian state mean?
The UK has consistently argued that the issue of whether Israel has committed genocide was a matter for the courts. Israel is fighting a case at the International Court of Justice in The Hague in which the country is accused of genocide.
But some opposition leaders, including Zack Polanski for the Green Party, and the Liberal Democrats' Sir Ed Davey have specifically referred to the situation in Gaza as genocide.
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On Tuesday, the UN Independent International Commission of Inquiry on the Occupied Palestinian Territory released a report, claiming: "It is clear that there is an intent to destroy the Palestinians in Gaza".
It said Israel's actions meet the criteria set down for defining a genocide.
The UK government has said its official position was it "has not concluded that Israel is acting with that [genocidal] intent".
Israel is currently undertaking a major ground offensive in Gaza, with thousands forced to flee from Gaza City in recent days.
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