The British government has received advice from its own lawyers declaring that Israel has infringed international humanitarian law in Gaza but has neglected to make it public, according to a leaked recording obtained by the Observer.

The comments made by the Conservative chair of the House of Commons select committee on foreign affairs, Alicia Kearns, at a Tory fundraising event on March 13 are in contrast with frequent ministerial denials and evasions on the topic.

On Saturday night, Kearns, a former Foreign Office and Ministry of Defense officer who has regularly challenged ministers, notably foreign secretary David Cameron, on the legal advice they have received, stuck by her statements and called for the government to come clean.

“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,” she said. “Transparency at this point is paramount, not least to uphold the international rules-based order.”

The disclosure will place Lord Cameron and Prime Minister Rishi Sunak under severe strain because any such legal advice would mean the UK had to suspend all arms supplies to Israel without delay.

Legal experts stated that refusing to do so would risk putting the UK in breach of international law itself, as it would be perceived as aiding and abetting war crimes by a government it was exporting guns to.

Answering questions during an “evening drinks reception” organized by the West Hampstead and Fortune Green Conservatives in London, Kearns said: “The Foreign Office has received official legal advice that Israel has broken international humanitarian law, but the government has not announced it.

“They have not said it; they haven’t stopped arms exports. They have done a few very tiny sanctions on Israeli settlers, and everyone worldwide believes that settlements are illegal, that they shouldn’t be doing what they’re doing, and the methods in which they have continued and the money that’s been put in.”

Kearns told the group that both she and Cameron believed passionately in Israel’s right to defend itself. “But the right to self-defence has a limit in law. It is not limitless,” she said, going on to suggest that Israel’s actions put its and the UK’s long-term security at risk.

“Some of the ways in which Israel is prosecuting this are making their long-term security less certain. It is making our long-term security less certain. I’m shocked that our national threat level has not gone up. And it kills my heart because I know it could be done differently.”

The British barrister and judge Sir Geoffrey Nice, who was the lead prosecutor during former Serbian president Slobodan Milošević’s trial from 2002 to 2006, said he would not be at all shocked if such advice had been offered by government attorneys and advocated for it to be made public.

Nice said: “A warring party becomes unlawful if it cannot show that its actions have been proportionate. It would not be shocking if there had been guidance to that effect from the Foreign Office’s lawyers.”

Were that to be the case, he stated that “at the very least, that would mean the UK would have to look at the whole issue of arms sales to Israel. It puts you in the area of assisting and abetting. It pushes you into very unpleasant areas.”

He added: “Countries supplying arms to Israel may now be complicit in criminal warfare. The public should be told what the counsel says.”

The UK’s weaponry exports to Israel totaled £42 million in 2022, a sum characterized by defence secretary Grant Shapps as “relatively small.”.

But former Lord Chancellor Charles Falconer claimed a legal finding that Israel had infringed international law would also bar the UK from sharing intelligence with Israel.

“Governments that abide by the rule of law cannot ignore mounting evidence of breach, which would then put those governments in breach if they continued assisting,” he said.

In a session of the foreign affairs select committee in January, Cameron was asked directly by Kearns whether “you have never had a piece of paper put in front of you by a Foreign Office lawyer that says that Israel is in breach of its international humanitarian commitments under international humanitarian law”.

Cameron remarked, “I cannot recall every single bit of paper that has been put in front of me… I don’t want to answer that question.”

He then remarked that “if you are asking me whether I am worried that Israel has taken action that might be in breach of international law, yes, of course I am worried about that. That is why I contact the Foreign Office attorneys when offering this guidance on arms exports.”

Other UK ministers have previously maintained that Israel has abided by international law. In late November, business secretary Kemi Badenoch said on Sky News that “we’ve always said that Israel should abide by international law, and that appears to be what they have done… It looks like they have taken tremendous care to make sure that they’re remaining within the bounds of the law. We applaud them for that.”

Labor has consistently urged the administration to be candid about the legal advice it has received.

On March 22, David Lammy MP, the opposition foreign secretary, wrote to Cameron, calling on him to reveal legal advice on Israel’s compliance with international humanitarian law.

On 26 March in the House of Commons, Lammy asked the minister for development and Africa, Andrew Mitchell MP, if the foreign secretary had received legal advice, saying there was a clear risk that items licensed by the UK might be used to commit or facilitate a serious violation of international humanitarian law. Mitchell said “no government does that”, adding later that “we do not disclose our internal legal advice”.

Last Monday, the International Court of Justice ordered Israel to provide unrestricted access to food supplies in Gaza, where enormous numbers of people are facing imminent hunger. Cameron has often underlined his unhappiness at Israel’s stance in barring aid from crossing the border into Gaza.

The battle began on October 7 after Hamas conducted an offensive within Israel that killed more than 1,100 Israelis, mostly civilians, and took roughly 250 people captive.

Israeli military operations in Gaza have resulted in the deaths of more than 32,000 individuals, the majority women and children, according to local health authorities.

A Foreign Office spokesperson said: “We keep advice on Israel’s adherence to international humanitarian law under review, and ministers act in accordance with that advice, for example, when considering export licences. The content of the government’s guidance is confidential.”