The UK is breaching international law by continuing to arm Israel, Rishi Sunak has been warned.
Three former Supreme Court justices are among over 600 lawyers and academics who have signed a 17-page letter to warn the present situation in Gaza is "catastrophic".
The government has been facing growing calls to suspend arms exports to Israel after three British aid workers were killed in an airstrike.
John Chapman, Jim Henderson and James Kirby were among seven World Central Kitchen aid workers who died when a convoy they were travelling in was hit on Monday - in what the Israel Defence Forces has called a "grave mistake".
The letter warns that - given the International Court of Justice's opinion that there is a plausible risk of genocide - the UK government is legally obliged to act in preventing it.
The signatories said: "While we welcome the increasingly robust calls by your government for a cessation of fighting and the unobstructed entry to Gaza of humanitarian assistance, simultaneously to continue... the sale of weapons and weapons systems to Israel... falls significantly short of your government's obligations under international law."
Ministers are also urged to work towards a ceasefire and sanction "individuals and entities who have made statements inciting genocide against Palestinians".
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Their letter calls for funding to be restored to the aid agency UNRWA, which was withdrawn amid allegations staff had participated in the 7 October attack orchestrated by Hamas.
And it said that "serious action" is needed to "avoid UK complicity in grave breaches of international law, including potential violations of the Genocide Convention".
The signatories include the Supreme Court's former president Lady Hale as well as former justices Lord Sumption and Lord Wilson.
Labour's shadow foreign secretary David Lammy said the government should publish any legal advice it has regarding whether Israel had breached international law - and suspend arms sales if there was a risk weapons could be used in "a serious breach of international humanitarian law".
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The Liberal Democrats and the SNP have gone further by calling for arms exports to be blocked.
Lib Dem leader Sir Ed Davey said suspending arms sales would "send a powerful signal from a close ally of Israel that we think this is unacceptable" - while Humza Yousaf, the Scottish first minister and leader of the SNP, said the UK risked being "complicit in the killing of innocent civilians".
The government does not directly supply Israel with weapons, but does grant export licences for British companies to sell arms to the country.
The US remains by far the largest supplier of weapons to Israel, with Foreign Office minister Andrew Mitchell recently telling MPs that UK exports accounted for just 0.02% of Israel's military imports.
The UK has twice suspended licences to sell arms to Israel in past decades - but the prime minister has so far resisted calls to do this again.
Speaking to The Sun, Mr Sunak said the UK has a "very careful export licensing regime that we adhere to".
He said: "There are a set of rules, regulations and procedures that we'll always follow, and I have been consistently clear with Prime Minister Netanyahu since the start of this conflict that while, of course, we defend Israel's right to defend itself and its people against attacks from Hamas, they have to do that in accordance with international humanitarian law, protect civilian lives and, sadly, too many civilians have already lost their lives."
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