David Lammy’s announcement of a suspension on 30 arms export licences to Israel finally returns some credibility to the UK’s arms control rules, but its obvious incoherence may leave the Labour government in a position that is unsustainable.

The UK government’s own guidance has long been clear. Britain’s position is that it will “not issue export licences if there is a clear risk that the items might be used to commit or facilitate serious violations of international humanitarian law”.

Yet, in practice, the so-called clear risk test has been anything but transparent. The high level of casualties in Gaza – 40,786 according to Gaza health authorities, the majority civilian, at the last count – includes over 100 recorded as killed in at least three individual bombing attacks.

It raises obvious questions about whether they are war crimes because of a sheer lack of proportionality, even though Israel says it is targeting Hamas fighters and leaders who are embedded within the Palestinian population, and that some civilian casualties are tragically inevitable.

Leaks about the number of civilians the Israel Defense Forces (IDF) are willing to accept as collateral damage are higher than those Nato militaries are generally considered prepared to accept. One report from +972 magazine said it was considered acceptable by the IDF to kill 100 civilians in an effort to eliminate a Hamas battalion or brigade commander.

Nevertheless, the previous Conservative government repeatedly concluded that Britain’s modest arms sales to Israel (£42m in 2022) were perfectly legitimate. A similar approach was taken during Saudi Arabia’s long air campaign against Houthi rebels in Yemen where civilians were killed repeatedly, but UK arms sales continued.

Lammy recognised on Monday that there “does exist a clear risk” that British munitions could be used to facilitate a “serious violation” of international law – before going on to exempt parts for F-35 fighters, which are used by Israel during its Gaza bombing campaign. Though assembled by US defence supplier Lockheed Martin, UK manufacturers supply a significant 15% of its components.

F-35 components going directly to Israel for certain would be banned, but the UK government claim is that is impossible to separate out for certain what is going to jets for Israel and what is not, a claim at odds with the high levels of control displayed by western governments over arms supplied for Ukraine.

If there was any doubt about the importance of F-35s, Information, a Danish news site, reported that one of the jets was engaged in the 13 July strike on the al-Mawasi ‘safe zone’ in southern Gaza, which the IDF said had killed Hamas leader Mohammed Deif. Reports said that at least 57 people were killed – and perhaps as many as 90.

However, there is a more fundamental incoherence in Lammy’s position. The grounds he cited for halting 30 arms export licences was not how Israel might have used them in bombing Gaza, but rather separate concerns about whether enough humanitarian relief was being permitted and the mistreatment of Palestinian detainees.

On the critical point, whether there was a clear risk that the intensity of the assault on Gaza was legitimate or not, the decision was fudged. “In many cases, it has not been possible to reach a determinative conclusion on allegations regarding Israel’s conduct of hostilities,” he said, partly because of lack of evidence from Jerusalem.

Western governments struggle, and often refuse, to accept claims about civilian harm caused by bombing or other military activity without conducting a level of investigation – typically via an on the ground inspection – that is practically impossible when a war is ongoing. But that does not mean it is impossible.

Local groups, specialist investigators and media organisations are able to compile consistent and accurate reports about incidents of bombing, list the fatalities involved and raise legitimate questions about whether such attacks may amount to a war crime. Airwars, a conflict monitor, has named 3,000 Palestinians killed in Gaza in almost 350 separate episodes during the first 17 days of the war.

Politically, it is hardly surprising that Labour have sought a fudged middle ground. Its aim is to register diplomatic concern about the conduct of the war with Hamas, ahead of party conference later this month, while trying to make clear, as the defence secretary, John Healey, did on Tuesday morning, that the UK would be ready to help defend Israel if it came under drone and missile attack from Hezbollah or Iran.

But the reality is that the UK’s guidelines for arms sales are simpler than the complexities of realpolitik. On Tuesday morning, lawyers representing al-Haq, a Palestinian human rights group and the Global Legal Action Network, who are challenging the legality of Britain’s arms exports, said they would continue their case – refocusing on F-35 parts and the overall use of munitions in Gaza.

The debate is far from concluded.