Keir Starmer’s most senior legal adviser has intervened in the controversial decision on whether to ban British arms sales to Israel because Israeli authorities find it difficult to distinguish between “offensive” and “defensive” weapons, the Guardian has learned.

According to sources, Attorney General Richard Hermer has told U.S. State Department officials that he will not agree to a ban on certain arms sales until it can be determined with certainty which weapons could be used to violate international humanitarian law.

The main reason for the delay in the decision is said to be legal disputes at the top of the government, which have become even more delicate in recent weeks given the escalation of the Middle East crisis.

A State Department spokesman declined to comment on Hermer’s role, but said: “This government is committed to upholding international law. We have made it clear that we will not export goods if they could be used to commit or facilitate serious violations of international humanitarian law.”

“The Secretary of State initiated an ongoing review process on day one of his term to determine Israel’s compliance with international humanitarian law. We will provide an update once that review process is complete.”

David Lammy has launched an inquiry into whether Britain should continue to sell arms to Israel as it continues its assault on Gaza. The foreign secretary argued for banning the sale of “offensive” weapons but allowing arms manufacturers to continue to supply “defensive” weapons, saying such a move would enable Israel to defend itself.

Although Britain only exports about £18 million worth of military equipment to Israel each year, the Israeli government reacts sensitively to any suggestion that Britain believes the country has violated international humanitarian law.

Benjamin Netanyahu is said to be angry about Lammy’s decision to drop Britain’s appeal against the International Criminal Court’s arrest warrant against him. The Israeli prime minister is now closely monitoring the outcome of the British arms review.

The decision has taken on even more significance in recent days after Israel launched airstrikes on Lebanon while Hezbollah carried out a drone and missile attack on northern Israel, sparking fears of a wider Middle East conflict.

Lammy was due to announce the results of his inquiry before MPs’ summer recess, but the decision has been delayed because senior government figures are unsure whether they can defend the offensive/defensive distinction in court.

Officials are examining each individual type of weapon to find out what purpose it was used for. Hermer, an ally of Prime Minister Keir Starmer since they worked as lawyers, has told officials they need to be sure that any weapon that continues to be sold has not been used to violate international humanitarian law.

In addition, Starmer is expected to take an active role in the decision-making process, although the review is officially carried out by the Foreign Office.

The authorities are keen to avoid a repeat of 2019, when an appeals court ruled that British arms sales to Saudi Arabia were illegal and that ministers had failed to properly investigate whether the weapons had been used to violate human rights laws in Yemen.

Last week, a group of human rights lawyers filed a lawsuit in the Supreme Court accusing the government of acting irrationally by refusing to ban arms sales. As part of their lawsuit, they submitted more than 100 pages of witness testimony alleging that Palestinians were tortured, left untreated in hospitals and unable to escape heavy bombing.

“International humanitarian law is vague, but it says we have to prove we have considered all options,” said a person familiar with the process. “So the biggest obstacle here is legal, not diplomatic.”

While the review is ongoing, the government has apparently stopped issuing new licenses for arms sales to Israel. Exporters applying for new licenses are reportedly receiving messages from the Ministry of Economy and Trade saying that applications are on hold until the review is completed.

Nevertheless, the delay in the investigation has caused a stir in parts of the British government. Earlier this month, a British diplomat in Dublin resigned from his post because ministers had not yet banned arms sales to Israel. Mark Smith told the BBC that he believed Israel was committing “war crimes in plain sight”.