More than 300 Foreign Office staff have been told to consider resigning after they wrote a letter over fears the government had become complicit in Israel’s alleged war crimes in Gaza.

It is the fourth internal letter from staff about the offensive in Gaza, which started in October 2023 in response to Hamas’s deadly attack on Israel.

In their letter of 16 May the staff, from embassies around the world and at various levels of seniority, questioned the UK’s continued arms sales and what they called Israel’s “stark … disregard for international law”.

The Foreign Office said it had systems for staff to raise concerns and added the government had “rigorously applied international law” in relation to the war in Gaza.

The reply to the letter was sent by the permanent under-secretary, Oliver Robbins, and Nick Dyer, the second most senior civil servant in the Foreign Office. They told the signatories: “If your disagreement with any aspect of government policy or action is profound, your ultimate recourse is to resign from the civil service. This is an honourable course.”

The reply did not address the substantive complaints by staff.

The letter, first reported by Novara Media, said: “In July 2024, staff expressed concern about Israel’s violations of international humanitarian law and potential UK government complicity. In the intervening period, the reality of Israel’s disregard for international law has become more stark.”

It also cited the killing by Israeli forces of 15 humanitarian workers in March and Israel’s suspension of all aid to Gaza in the same month, “leading many experts and humanitarian organisations to accuse Israel of using starvation as a weapon of war”.

It said the UK government’s position had contributed to the “erosion of global norms”, citing continued weapons exports and the visit to London in April by Israel’s foreign minister, Gideon Sa’ar, “despite concerns about violations of international law”. The Foreign Office described Sa’ar’s visit as private, even though he met the foreign secretary, David Lammy.

The staff letter added: “Supported by the Trump administration, the Israeli government has made explicit plans for the forcible transfer of Gaza’s population. Israel’s finance minister, Bezalel Smotrich, recently stated that he envisions Gaza being completely destroyed. The Israeli security cabinet has approved plans to expand its military offensive to include the capture of Gaza. Any such transfer would be in clear breach of the fourth Geneva convention.”

The letter called on the UK government to uphold international law by promoting accountability at the international criminal court and implementing the judgments of the international court of justice. It also called for a suspension of the free trade deal, a complete suspension of arms sales, the publication of legal advice to ministers, an evidence-based review of the UK government’s response to the conflict, and encouragement of a Foreign Office speak-up culture by establishing an internal challenge mechanism.

The staff said there would be an impact on the UK’s reputation if it maintained its existing relationship with Israel.

In response, Robbins and Dyer said the department welcomed “healthy challenge” as part of the policymaking process and had already set up a “bespoke challenge board” and regular listening sessions with employees to hear concerns in this policy area.

They wrote that staff were entitled to their personal views, but added it “might be helpful” to “remind” them of mechanisms available to those uncomfortable with policy. It went on to list ways staff could raise issues, but added that resignation was an “ultimate recourse” and “honourable course” for those with profound disagreements over government policy.

“The bargain at the heart of the British civil service is that we sign up to deliver the policies of the government of the day wholeheartedly, within the limits imposed by the law and the civil service code,” it said.

The UK government’s position is that Israel is “at risk” of breaching humanitarian law, the threshold for barring arms exports, but it says it is for international courts to determine if breaches of international law have occurred, which will not be fully determined for many years.

Senior Foreign Office ministers are due to face questions by the business select committee over why the government is continuing to sell parts and components to the F-35 programme without placing a condition that the parts are not sent on to Israel.

The UK does not sell directly to Israel, and claims it has no option but to supply the parts or see the whole F-35 programme grind to a halt, affecting Nato operations defending Europe.

The carve-out of F-35s from the ban on UK arms being sold to Israel, imposed in September, is being tested in the high court by the NGOs Global Legal Action Network and Al-Haq.

The Foreign Office in its court submissions, likely to be the subject of cross-examination by the business committee, said it had determined Israel was not committing genocide in Gaza, which appears to contradict the stance that only the UK courts can make such a ruling. It also said it could not take a position on specific attacks by Israel since it did not have definitive evidence.

In September, Lammy announced the suspension of about 30 arms exports licences to Israel, and said the remaining licences were not relevant to the war in Gaza – although the government admits some of the licences allow exports to the Israel Defense Forces.

Israel has denied committing war crimes in Gaza, saying its actions are proportionate and necessary to eradicate Hamas, which it says uses hospitals and school premises to protect itself.

The former Labour leader Jeremy Corbyn has gathered the names of 50 MPs backing his call for an independent public inquiry into UK involvement in military operations in Gaza. Corbyn has been among MPs pressing ministers to explain why RAF jets from the UK base in Akrotiri in Cyprus regularly fly over Gaza. More than 300 surveillance flights have been recorded, allegedly in search of hostages held by Hamas.

Questions are also being asked if Israel acted lawfully by intercepting the ship Madleen in international waters, which among its crew, were Greta Thunberg and 11 other campaigners seeking to highlight the blockade of food into Gaza.

The Freedom Flotilla Coalition , the group operating the UK-flagged Madleen, said all 12 campaigners were “being processed and transferred into the custody of Israeli authorities”. The Foreign Office has not commented.