The Government is facing calls to halt the sales of arms to Saudi Arabia amid claims British-made weapons could have been used in air strikes which have left thousands of civilians dead in Yemen's bloody civil war.
Human rights organisations have warned the UK is breaking national, EU and international law by supplying weapons to the Saudis while their warplanes carried out bombing raids in support of the Yemeni government.
However, the Foreign Office has insisted that Britain is not in breach of its international obligations and that it operated one of the most rigorous and transparent arms control regimes in the world.
According to a report by Amnesty International and Saferworld, the UK has issued more than 100 arms export licences worth more than £1.75 billion to Saudi Arabia since March, when its air force began attacking Houthi rebels trying to overthrow the Yemeni government.
In that period more than 5,800 people have been killed, according to the United Nations, while tens of thousands have been injured and 2.5 million forced to flee their homes to escape the fighting.
A legal opinion commissioned by Amnesty and Saferworld concluded any permit of the transfer of weapons to Saudi Arabia "in circumstances where such weapons are capable of being used in the conflict in Yemen" and there was no restriction on their end use "would constitute a breach by the UK of its obligations under domestic, European and international law".
It said the Government could be deemed to have "actual knowledge ... of the use by Saudi Arabia of weapons, including UK-supplied weapons, in attacks directed against civilians and civilians objects, in violation of international law" since at least May 2015.
The opinion was drawn up by Professor Philippe Sands QC, Professor Andrew Clapham and Blinne Ni Ghralaigh of Matrix Chambers.
Amnesty International UK director Kate Allen said it confirmed their view that the sale of arms to Saudi Arabia was "illegal, immoral and indefensible".
"The UK has fuelled this appalling conflict through reckless arms sales which break its own laws and the global Arms Trade Treaty it once championed," she said.
"Thousands of civilians have been killed in Saudi-led air strikes, and there's a real risk that misery was 'made in Britain'."
For Labour, shadow international development secretary Diane Abbott called for an immediate halt to British arms sales to Saudi Arabia.
"There needs to be an inquiry into what happened to these arms," she told Channel 4 News. "For too long the British arms agency has skirted on the edge of what is legal and legitimate in relation to arms control."
A Government spokeswoman said: "The UK is satisfied that we are not in breach of our international obligations.
"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 in Yemen. We monitor the situation carefully and have offered the Saudi authorities advice and training in this area."