Saudi Arabia handed over $1.5 billion to UK arms' dealers for bombs in just three months in 2015.
It came as Riyadh and its Gulf allies launched a massive air and land offensive against Houthi rebels in Yemen, which has come under scrutiny from human rights groups due to massive civilian losses.
UK checks on how the missiles, bombs and rockets would be used also appear to be lax or non-existant.
The deals ramped up sales of British weapons to the Gulf by over a hundred times, The Independent revealed, from just $12.7 million to $1.5 billion.
Rockets and bombs
Riyadh appears to have purchased bombs, rockets and missiles, which could have been used on Houthi targets - and inevitably lead to civilian loss of life.
Yemenis are on the brink of an outbreak of mass starvation and disease, medics have said, as a blockade of civilian areas by both parties continues.
|The UK arms deal was dated between June 2015 and September|
The UK arms deal was dated between June 2015 and September, just when peace talks to end the crisis were taking place in Geneva and four months after a visit to Riyadh from Prince Charles.
The licence carried an ML4 certificate - which are used for bomb, rocket and missile exports - and the deal would have been signed off by British ministers, The Independent said.
London has defended both UK arms sales to Riyadh and the Saudi kingdom's war in Yemen.
Foreign Minister Phillip Hammond said the UK had "a very robust export licensing process".
The news comes as Hammond admitted to UK military involvement in the Saudi-led campaign.
British troops have helped identify targets in Yemen for Gulf war planes, he admitted.
"We do have a military presence in Saudi Arabia and we are working with the Saudi Arabians to ensure the following of correct procedures to avoid breaches of international humanitarian law," Hammond said on Tuesday.
"That is to ensure that target sets are correctly identified, that processes are correctly followed to ensure that only targets that are legitimate military targets are struck."
He said British soldiers reported "no evidence of deliberate breach of international humanitarian law".
It is not the first time Hammond has defended Saudi Arabia's participation in the war in Yemen, which has left over 2,800 civilians dead.
|The important thing is that [bombs] are being used legally in an international armed conflict
- UK Foreign Minister Phillip Hammond
Although both pro and anti-government Yemeni forces have been accused of killing civilians Saudi's air assault has also been held under scrutiny.
NGOs and human rights groups say that air raids take place in populated areas, and have destroyed homes, hospitals and historic sites.
In November, the UK foreign minister again defended UK arms sales to Riyadh.
"What matters is that they are used legally in compliance with international humanitarian law and we monitor that very carefully," he said.
"The important thing is that they are being used legally in an international armed conflict. There have been accusations of breaches of international humanitarian law."
He also said that Saudi Arabia has denied any breaches of international law when the arms sales went through, although there appears to be no checks on the claims at the time the billion dollar deal was made.
"Obviously that denial alone is not enough. We need to see proper investigations," said Hammond.
"We need to work with the Saudis to establish that international humanitarian law has been complied with."
He said that the UK government would respond if Saudi was found to have broken international law.
"We will then find that we cannot licence additional shipments of weapons."