Human rights groups have said that Britain sold more than £1bn worth of arms to Saudi Arabia over three months last summer in a period which coincided with a surge of airstrikes by a Saudi-led coalition against Houthi rebels in Yemen.
Saferworld and Amnesty International commissioned the views of lawyers which concluded that British arms sales to the kingdom, in the context of Riyadh's Yemen campaign, broke national, EU and international law.
They said BIS documents show arms sales from July to September 2015 for the export category that covers missiles, rockets and bombs amounted to £1,066,216,510 and were sold under five separate licences. This is compared with arms sales of only £9m for the previous three months.
A Saferworld spokesperson told The Guardian that Britain armed a Saudi Air Force that "has bombed hospitals, schools, markets, grain warehouses, ports and a displaced persons camp and helped to turn Yemen into a living nightmare. That's a 100-fold increase over the previous three months."
Prime minister David Cameron has strongly defended British ties with Saudi Arabia saying that the kingdom is a significant opponent of Islamist militants Daesh.
Allan Hogarth, Amnesty International UK's head of policy, said: "These figures are deeply worrying, showing that the UK continued to dispatch huge amounts of weaponry to Saudi Arabia despite overwhelming evidence that the Saudi war machine was laying waste to Yemeni homes, schools and hospitals.
However a government spokesperson told the newspaper: "We operate one of the most rigorous and transparent arms export control regimes in the world with each licence application assessed on a case by case basis, taking account of all relevant information, to ensure compliance with our legal obligations. No licence is issued if it does not meet these requirements.
"We regularly raise with Saudi Arabian-led coalition and the Houthis, the need to comply with international humanitarian law (IHL) in Yemen. We monitor the situation carefully and have offered the Saudi authorities advice and training in this area."