The British government has received advice from its own lawyers stating that Israel has breached international humanitarian law in Gaza but has failed 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 13 March are at odds with repeated ministerial denials and evasion on the issue.

On Saturday night, Kearns, a former Foreign Office and Ministry of Defence official, who has repeatedly pressed ministers, including foreign secretary David Cameron, on the legal advice they have received, stood by her comments 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 revelation will place Lord Cameron and prime minister Rishi Sunak under intense pressure because any such legal advice would mean the UK had to cease all arms sales to Israel without delay.

Legal experts said that not to do so would risk putting the UK in breach of international law itself, as it would be seen as aiding and abetting war crimes by a country it was exporting arms to.

Answering questions at an “evening drinks reception” hosted 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 small sanctions on Israeli settlers and everyone internationally is agreed that settlers are illegal, that they shouldn’t be doing what they’re doing, and the ways in which they have continued and the money that’s been put in.”

Kearns told the gathering that both she and Cameron believed strongly 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 is making their long-term security less certain. It is making our long-term security less certain. I’m amazed that our national threat level has not gone up. And it breaks my heart because I know it could be done differently.”

The British barrister Sir Geoffrey Nice, who was the lead prosecutor at former Serbian president Slobodan Milošević’s trial from 2002 to 2006, said he would not be at all surprised if such advice had been given by government lawyers and called 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 surprising if there had been advice to that effect from the Foreign Office’s lawyers.”

Were that to be the case, he said that “at the very least that would mean the UK would have to look at the whole issue of arms sales to Israel. It takes you into the area of aiding and abetting. It takes you into to very difficult areas.”

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

The UK’s arms exports to Israel amounted to £42m in 2022, a figure described by defence secretary Grant Shapps as “relatively small”.

But former lord chancellor Charles Falconer said a legal assessment that Israel had broken international law would also prevent the UK sharing intelligence with Israel.

“Governments who 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 stated that “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 later said 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 consult the Foreign Office lawyers when giving this advice on arms exports.”

Other UK ministers have previously claimed 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 great pains to make sure that they’re staying within the confines of the law. We applaud them for that.”

Labour has repeatedly called for the government to be transparent about the legal advice it has received.

On 22 March, David Lammy MP, the shadow foreign secretary, wrote to Cameron, calling on him to publish the 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 do[es] that”, adding later that “we do not disclose our internal legal advice”.

Last week, the international court of justice ordered Israel to allow unimpeded access of food aid into Gaza, where huge numbers of people are facing imminent starvation. Cameron has repeatedly voiced his frustration at Israel’s action in blocking aid from crossing the border into Gaza.

The war began on 7 October after Hamas launched an attack inside Israel that killed more than 1,100 Israelis, mostly civilians, and took about 250 people hostage.

Israeli military attacks on Gaza have resulted in the death of more than 32,000 people, 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 advice is confidential.”