A United Nations panel of experts has uncovered fragments of British-made laser guidance missile systems at an air raid site in Yemen in a strike that it concluded breached international humanitarian law.

The attacks took place in September 2016, a month after the then foreign secretary Boris Johnson said he was content to allow the export of weapons systems to Saudi Arabia in the expectation they would be used in Yemen.

A guidance unit for a “high explosive” bomb – stamped with the name of a Brighton based company, EDO MBM Technology Ltd – were found at the site in the Yemen capital Sana’a after four bombs were dropped on the site at 12.45am on 13 September.

Missile parts from the same British factory – ultimately owned by the US arms supplier L3 Harris – were also found by the UN experts at the Alsonidar complex following a second air strike nine days later, where a water pump factory and a former tube maker were located.

Though the UK has licensed at least £4.7bn of exports to Saudi Arabia since the country became embroiled in the war in Yemen, the UN documentation demonstrates that British technology has been deployed in a conflict where the Saudi-led coalition has been repeatedly accused of indiscriminate bombing.

British arms sales to Saudi Arabia of equipment that could be deployed in Yemen were held to be unlawful in June by the court of appeal in London, because ministers had failed to conduct a proper impact assessment of the bombing on civilians.

Dr Anna Stavrianakis, a senior lecturer in international relations at the University of Sussex, said the UN report showed that such an assessment could have taken place. “This revelation is a damning indictment of a policy that is reckless in its disregard for civilian harm.”

At the time, a spokesman for the Saudi-led coalition said that warplanes had hit the Alsonidar plant because it “is now becoming a military manufacturing unit specialised in producing pipes Houthis use to assemble local-made missiles.”

But reporting to the UN security council, the panel concluded “there is insufficient evidence to demonstrate that the factory complex had become a legitimate military objective” because there was no evidence that any hardware was manufactured on the site.

The tube maker on the site had not been operational since 2014, and the panel said the only mitigating factor was that there were no casualties in the bombing because the attacks took place shortly after midnight.

The UN accounts were made publicly available in January 2018, following the panel investigation, though they had remained largely unnoticed.

Ministers have said they would appeal the ruling in the upreme court but the former international trade secretary, Liam Fox, told parliament that arms exports to Saudi Arabia would be suspended pending an official review.

“The government’s emphasis on the diplomatic, strategic and economic benefits of arms sales are ringing increasingly hollow,” Stavrianakis said. “Its commitment to international law is repeatedly revealed to be superficial.”