Supplying arms to Israel while simultaneously providing humanitarian aid to Gaza is “intellectually and morally incoherent”, the head of Oxfam GB has said.

According to Gaza’s health ministry, more than 37,000 Palestinians, mostly civilians, have been killed in the Israeli military offensive on Gaza since the 7 October attacks by Hamas. Israel and Hamas have both been accused of breaking international law.

Nevertheless, UK ministers have rejected calls to suspend arms exports to Israel, prompting a legal challenge, in which Oxfam was on Thursday granted permission to formally intervene.

In an interview with the Guardian, Halima Begum, who took over as Oxfam GB chief executive in April and recently returned from a work trip to Israel and the occupied West Bank, said the UK’s stance does not make sense.

“Whether you say they are components or whole weapons [being sold] is a moot point, because individual components collectively constitute these devices that are killing so many innocent people. The UK needs to stop selling these arms. The government can’t simultaneously give humanitarian aid and talk about its aspirations for peace in the region, then also ship bombs – it’s intellectually and morally incoherent.

“That the law doesn’t prevent the trade seems immaterial. If you knowingly sell weapons that are being used to kill thousands of innocent children and their parents, why would you continue?”

Despite being unable to enter Gaza because of the assault on Rafah, Begum said she was left “shell shocked” after hearing first-hand accounts of the humanitarian crisis from Palestinian colleagues evacuated from the territory.

The former ActionAid CEO said that she visited Islam’s third holiest site, the al-Aqsa mosque in Jerusalem, during her trip, and prayed there for the people in Gaza and the Israeli hostages, 120 of whom are still being held in the Palestinian territory, although at least 40 of those are believed to be dead.

She said that there was precedent for the UK and US refusing to arm Israel, citing both governments’ decision to do so in 1982, after Israel’s invasion of Lebanon. (Tony Blair also did so in 2002).

Begum said: “Margaret Thatcher halted weapons exports to Israel during the Lebanon war. Ronald Reagan suspended shipments of cluster munitions in July 1982 and he was reportedly so shocked by images of dismembered Palestinian children in a bombardment on 12 August that he warned Israeli PM Menachem Begin “our entire future relations are at stake if this continues”. Israel ordered a complete ceasefire before the day was out. So it wouldn’t be the first time a British or US government has drawn a moral line.”

“I utterly condemn the taking of any hostages and any atrocity committed by either side. Now though, we have 37,000 Palestinians dead. Gazan children are being bombed, suffering from malnutrition and facing potential famine and the UK still can’t constrain the Israeli military. It defies belief we’d support this action, our humanity seems to be seeping away.”

Government figures published last week showed that the UK issued 108 licences to Israel between the 7 October attacks and 31 May and that no arms export licence application has been rejected or revoked during that timeframe.

Begum said the global south was fairly united on what needs to happen regarding Gaza and it seemed to be “only western leaders that don’t see what is morally the right thing to do”. She expressed concern that the west’s stance jeopardised prospects for a post-conflict international order based on humanitarian law and principles of human rights.

“If you have a friend and their behaviour is atrocious, you’re still able to say, ‘Look, as friends, you shouldn’t be doing that,’” she said. “That doesn’t mean you can’t offer your support to a friend. I feel as though that whole construction around Israel’s right to self-defence, every country has a right to defend themselves, but not at the cost of humanitarian law being ripped up in shreds, without any reference to human rights on the ground.”

The judicial review against arms sales to Israel is being brought by the Palestinian human rights organisation Al-Haq and the UK-based Global Legal Action Network (Glan). Oxfam previously intervened in a case brought by Campaign Against Arms Trade (CAAT) which successfully argued that British arms sales to Saudi Arabia, which was engaged in war in Yemen, were unlawful.

The UK government declined to comment.