Britain's support for Israel depends on it abiding by international humanitarian law, David Cameron said in a newspaper column on Sunday.
The foreign minister spoke days after an Israeli airstrike killed seven aid workers, including three Britons.
"Our backing is not unconditional," Lord Cameron wrote in The Sunday Times. "We expect such a proud and successful democracy to abide by international humanitarian law, even when challenged."
The British government has been a staunch ally of Israel since the Hamas attack on southern Israel on October 7 last year which triggered Israel's war to eliminate the militant Islamist group.
But Lord Cameron has hardened his language in recent months over the dire humanitarian situation in the Palestinian enclave.
Lord Cameron warned on Sunday of the risk of mass starvation unless Israel allowed more aid. On Saturday, Britain said it would supply a naval vessel to ship aid as part of an international effort.
In a statement on Sunday to mark six months since the initial Hamas attack, Rishi Sunak repeated his call for the Palestinian group to release Israeli hostages and for an immediate pause in fighting.
"We continue to stand by Israel's right to defeat the threat from Hamas terrorists ... but the whole of the UK is shocked by the bloodshed, and appalled by the killing of brave British heroes who were bringing food to those in need," he said.
Britain's government is also under pressure to publish its latest legal advice about Israel's conduct of the war in Gaza, which would potentially affect British arms exports.
Last week three former Supreme Court justices joined more than 600 members of the British legal profession in calling for the government to halt arms sales to Israel, saying it could make Britain complicit in genocide in Gaza.
Britain supplied £42 million of arms to Israel in 2022. In December, the government decided these exports should continue but would be kept under review.
Lord Cameron said on March 8 that a fresh judgement on that was underway and due in the "coming days".
David Lammy, the opposition Labour Party's would-be foreign minister, called for the government to publish a summary of its latest legal advice.
"I do have very real concerns that our obligations in relation to international humanitarian law and ... our export and licensing regime ... might have been breached," he told the BBC.
Deputy prime minister Oliver Dowden told Sky News that the government did not plan to publish legal advice but denied it was giving Israel "carte blanche".
"Of course, Israel has made mistakes and made big mistakes, and we should hold them to account for that," he said.
"It's right that we hold Israel to high standards. But I just think there's a bit of relish from some people about the way in which they are pushing this case against Israel."
