London (CNN) — Pressure is mounting on British Prime Minister Rishi Sunak to suspend the sale of arms to Israel following the deadly attack on a convoy of aid workers in Gaza.

Calls for Sunak to stop supplying Israel with weapons grew after an Israeli airstrike on Monday killed seven members of staff from World Central Kitchen, three of whom were British citizens.

The government is still waiting for legal advice from its lawyers on whether or not selling arms to Israel is in breach of international law. Sunak is also under pressure to publish any legal advice he has been provided with on whether or not the Israeli government has breached international law through its actions in Gaza.

A recording emerged at the weekend of Alicia Kearns, who chairs the UK Parliament’s Foreign Affairs Select Committee, saying she is convinced the government has already received advice that Israel’s actions are illegal and has declined to publish it.

Following the leak, Kearns stood by the report and said in a statement: “I remain convinced the government has completed its updated assessment on whether Israel is demonstrating a commitment to international humanitarian law, and that it has concluded that Israel is not demonstrating this commitment, which is the legal determination it has to make.”

The governing Conservative Party is historically supportive of Israel, but the killing of British citizens has shifted the domestic debate.

On Thursday, more than 600 lawyers, legal academics and former members of the British judiciary wrote to Sunak, warning him that “serious action” is needed to “avoid UK complicity in grave breaches of international law, including potential violations of the Genocide convention.”

“While we welcome the increasingly robust calls by your government for a cessation of fighting and the unobstructed entry to Gaza of humanitarian assistance,” the letter says, “simultaneously to continue (to take two striking examples) the sale of weapons and weapons systems to Israel and to maintain threats of suspending UK aid to UNRWA falls significantly short of your Government’s obligations under international law.”

The three main opposition parties, Labour, the Liberal Democrats and the Scottish National Party, have all called for the suspension of arms sales to Israel if government lawyers say it is illegal, and have demanded that Sunak outline why it has not happened yet.

Sunak has taken a firmer line in recent calls with Israeli Prime Minister Benjamin Netanyahu. According to a government readout of a call that took place on Tuesday, Sunak said he was “appalled by the killing of aid workers, including three British nationals,” and demanded a “thorough and transparent independent investigation into what happened.”

The Downing Street release claimed Sunak said “far too many aid workers and ordinary civilians have lost their lives in Gaza and the situation is increasingly intolerable,” and that he “reiterated that Israel’s rightful aim of defeating Hamas would not be achieved by allowing a humanitarian catastrophe in Gaza.”

Defense sales from the UK to Israel are “relatively small,” Defense Secretary Grant Shapps said earlier this week, when compared to the amount of weapons that come from the US and Germany. However, parliamentary estimates show that the UK has still licensed arms worth over $725m.

Diplomatically, it would be a significant step by a key Israeli ally and permanent member of the UN Security Council. It would also set Britain apart from a number of its allies, including the US and Germany, if it were to take the action unilaterally.

Suspending sales would be also politically difficult for Sunak domestically, as it’s likely not everyone in his own government or party would support him.

The family of one of the aid workers killed former Royal Marine James Henderson, have criticized the UK sale of arms to Israel. Speaking on their behalf, his brother told the Times of London that accountability was their “only hope of justice.”

“I don’t believe our government will hold the correct people to account, but I guarantee that our government will sell weapons to Israel, which may in turn be used to kill our fellow citizens.

“It’s hard to comprehend that.”

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