The UK has broken with the Biden administration on a significant part of their tightly coordinated policy towards Israel by announcing it is suspending some arms export licences to Israel because of a “clear risk” they may be used to commit or facilitate a serious violation of international humanitarian law. The Foreign Office said a two-month internal review had raised concerns about the way Israel had conducted itself in the conflict in Gaza and that the decision specifically related to concerns around the treatment of Palestinian detainees and the supply of aid to Gaza. No definitive conclusion has been reached about whether UK arms export licences have contributed to the destruction in the territory. But the scale of the devastation and the number of civilian deaths caused great concern, the Foreign Office said. The suspension, which is likely to cause tensions with the US government, covers components for military aircraft, helicopters, drones and targeting equipment. The UK foreign secretary, David Lammy, said it applied to 30 of the 350 existing arms licences, but would almost entirely exclude all UK components for the F-35 fighter jet programme, seen as a significant loophole by pro-Palestinian groups. F-35 components have been exempted, officials say, because they are part of a global programme and the UK does not have unilateral control of these components, which are sent to the US. They will, however, not be exempt on the rare occasion where the part is being sent directly to Israel. Lammy, aware of the sensitivity of the issue in Israel and the US, stressed his decision was taken more in sorrow than anger, adding the conclusion did not amount to a full arms embargo, and did not even go as far as the suspension of licences made by Margaret Thatcher in 1982. But the Israeli defence minister, Yoav Gallant, said he was deeply disheartened by the decision, adding: “This comes at a time when we fight a war on seven different fronts – a war that was launched by a savage terrorist organisation, unprovoked. At a time when we mourn six hostages who were executed in cold blood by Hamas inside tunnels in Gaza. At a time when we fight to bring 101 hostages home.” Israel’s foreign minister, Israel Katz, said he was “disappointed” by the British decision, adding it sent “a very problematic message to the terrorist organisation Hamas and its sponsors in Iran”. The Israeli prime minister, Benjamin Netanyahu, is already under political siege after a general strike and fury that his stance over the Gaza ceasefire terms may have contributed to the Hamas killing of six Israeli hostages last week. Lammy told the House of Commons that the suspension decision was based primarily on evidence concerning the treatment of Palestinian prisoners and restrictions on the supply of humanitarian aid to Gaza. He said Israel’s conduct of the war in Gaza including the wide-scale destruction of houses contributed to the assessment of a clear risk of a serious breach of international humanitarian law. He appeared anxious for the decision not to lead to a collapse in Anglo-Israeli relations. Describing himself as a liberal progressive Zionist, he said: “We have not – and could not – arbitrate on whether or not Israel has breached international humanitarian law. This is a forward-looking evaluation, not a determination of innocence or guilt. And it does not prejudge any future determinations by the competent courts.” In a brief summary of its legal advice the Foreign Office (FCDO) said it found “Israel could have done more reasonably to facilitate humanitarian access and distribution”. It said for example Israel should establish a speedier and more effective system for protecting humanitarian aid from military operations. The FCDO added: “It could also better resource security control procedures and adopt a less restrictive approach to dual-use items (those with both military and civilian uses).” The advice also said the amount of aid provided was not enough, even if it was sufficient to be essential to the population’s survival. On the maltreatment of Palestinian detainees, the summary found “the volume and consistency of these allegations suggest at least some instances of mistreatment contrary to international humanitarian law. Israel has launched investigations into these allegations.” It added that the sufficiency of those investigations was unclear, partly because Israel continues to deny access to places of detention for the International Committee of the Red Cross (ICRC). International humanitarian law requires such access “except for reasons of imperative military necessity, and then only as an exceptional and temporary measure”. “Israel has not provided satisfactory reasons,” the FCDO said. On the conduct of the war itself Lammy said: “Israel’s actions in Gaza continue to lead to immense loss of civilian life, widespread destruction to civilian infrastructure, and immense suffering,” but added: “In many cases, it has not been possible to reach a determinative conclusion on allegations regarding Israel’s conduct of hostilities, in part because there is insufficient information either from Israel or other reliable sources to verify such claims.” Lammy’s statement was not condemned by opposition MPs, who described it as carefully calibrated, although Sammy Wilson of the Democratic Unionist party said the only people who would be overjoyed by this decision would be Hamas. MPs on the left saw the decision as a start or the bare minimum considering the loss of 40,000 Palestinian lives. The move, which was coordinated between the FCDO, the business department and Richard Hermer, the attorney general, is likely to help Lammy overcome what may be a highly charged revolt on the floor of the Labour party annual conference. British companies sell a relatively small amount of weapons and components to Israel. Earlier this year, the government reported that military exports to Israel amounted to £42m in 2022. But it will cause strains with the Biden administration in the US, and some Republicans close to Donald Trump. Both have repeatedly said they see no basis in international humanitarian law to suspend arms exports. Joe Biden is under pressure from the pro-Palestinian wing in the Democrats to use more leverage in the forms of arms sales to force Netanyahu to make concessions in the ceasefire talks. In Europe only Belgium and Spain have taken the step of imposing an arms embargo, but Germany has refused. The UK government is also facing a growing number of domestic court challenges, including proceedings due to start on Tuesday brought by Global Legal Action Network and the Palestinian human rights organisation Al-Haq. Officials said Lammy and his aides had been given no access to the decision-making process on arms sales made by the previous Conservative government. But the clear implication is that Labour ministers will have reached a different decision on the basis of similar evidence. • This article was amended on 3 September 2024. An earlier version referred to the loss of 40,000 “civilian” lives; while the majority of the dead are civilians, the toll also includes Hamas fighters.