Hundreds of lawyers have called on the UK government to use “all available means” to stop the fighting in Gaza, including reviewing trade ties with Israel and imposing sanctions and travel bans on Israeli ministers.

Some 828 UK-based or qualified legal experts, among them former Supreme Court justices, signed a letter to Prime Minister Sir Keir Starmer on Monday.

They warned “genocide is being perpetrated in Gaza” from Israel’s blockade of food and aid and its new military offensive, which has killed hundreds of Palestinians there in the past fortnight.

Israel has strongly denied genocide allegations, claims which are also being examined by the International Court of Justice.

The lawyers’ letter comes amid a wave of increased criticism from Israel’s Western allies, condemning its latest military offensive, which began in mid-May, and the impact of its blockade. The blockade was in full effect for 11 weeks from 2 March.

Last week, the UK, France and Canada issued a joint statement condemning Israel’s “egregious” expansion of military operations in Gaza.

Israeli Prime Minister Benjamin Netanyahu accused the British, French and Canadian leaders of “siding with Hamas”, saying the offensive is aimed at freeing the remaining 58 hostages held by the group.

But UK Foreign Secretary David Lammy called the escalation “morally unjustifiable”. The UK last week suspended trade deal talks, summoned Israel’s ambassador and imposed fresh sanctions on West Bank settlers.

Netanyahu has declared Israel’s intention to take over the Gaza Strip and displace residents from the north to the south.

The operation has seen about a 100 Gazans killed per day in strikes in its first week, according to the territory’s Hamas-run rescue authorities and health ministry.

On Monday, the group of legal experts said in a detailed 36-page letter that Israel’s May offensive was “a grave violation” of international law, and also violated the Palestinian people’s legal right to self-determination.

The group of lawyers, legal experts and retired judges expressed their “deep concern over the worsening catastrophe” in Gaza and the occupied West Bank where violations of international law were “being committed and are further threatened” by Israel.

“First, genocide is being perpetrated in Gaza or, at a minimum, there is a serious risk of genocide occurring,” the legal experts wrote.

They said this had been caused by Israel’s blockade – and the trickle of aid now being allowed in was “gravely insufficient.”

They said the UK and all countries were legally obliged to “prevent and punish genocide”.

“The UK’s actions to date have failed to meet those standards”, they said, but they welcomed the “indication” of stronger action in last week’s statement.

The UK must immediately impose financial and immigration sanctions on Israeli ministers suspected of “unlawful conduct”, to help secure a ceasefire, the lawyers wrote.

The letter also called for the UK to review existing trade ties with Israel, impose trade sanctions on the country and also suspend the UK’s “2030 Roadmap” with Israel – an agreement between the UK and Israel on defence, technology and science, culture, the arts and other areas.

They called on the government to ensure it would adhere to its obligations as a member of the International Criminal Court to act on arrest warrants issued by the court. Netanyahu is currently wanted by the court for alleged war crimes in Gaza and breaches of international law, charges he emphatically denies.

The letter is the group’s latest since the war began in October 2023. Its first missive was issued weeks into the conflict, where it raised concerns about the UK government selling arms to Israel.

The legality of the UK’s decisions concerning arms sales is now being examined by the High Court of England and Wales.

Israel launched a military campaign to destroy Hamas in response to an unprecedented cross-border attack on 7 October 2023, in which about 1,200 people were killed and 251 others were taken hostage.

At least 54,056 people have been killed in Gaza since then, including 3,901 since Israel resumed its offensive on 18 March following a two-month ceasefire, according to the territory’s Hamas-run health ministry.

On Tuesday, Germany and Finland also spoke out, saying Israel must allow humanitarian aid into Gaza immediately.

“We must put pressure on Israel to ensure the aid truly reaches its target,” German Chancellor Friedrich Merz told reporters alongside Finnish Prime Minister Petteri Orpo in Finland.

“But it is also crucial that Hamas must not prevent humanitarian aid from arriving.”

Merz, who has strongly criticised Israel in recent days, said events in Gaza were “in no way acceptable”. He described the effect on Gazan civilians as “excessive” and called for an end to the killing and suffering.

“This is a terrible human catastrophe and we must be able to tackle it,” Orpo added.

Sweden on Tuesday also summoned the Israeli ambassador to its foreign ministry, and called on the country to “immediately ensure safe and unhindered humanitarian access to Gaza”.

The Swedish foreign ministry said in a statement that Israel had a right to defend itself but “the current way the war is waged is unacceptable”.