A campaign group is threatening to take the British government to court over the sale of arms to Saudi Arabia.
Campaign Against The Arms Trade (CAAT) says Britain is breaching international humanitarian law over the use of weapons in the conflict in Yemen.
CAAT said it would take legal action by issuing judicial review proceedings unless the government halts such sales.
The UK said it had stressed to the Saudis the importance of investigating claims of international law violations.
At least 5,870 people have been killed since March, when a Saudi-led coalition began a military campaign to restore Yemen's government and drive back a rebel group - known as the Houthis - and their allies.
Leigh Day, the law firm representing CAAT, wrote to the government in November, highlighting concerns over allegations of violations of the laws of war by the Saudi-led coalition.
In particular they pointed to reports by respected NGOs of the targeting of civilians and civilian objectives in coalition airstrikes.
The United Nations estimates that 60% of civilian deaths and injuries since the start of the campaign have been caused by coalition airstrikes.
"The UK has a very clear legal regime, and that regime says that the UK won't provide licences for arms exports if there is a clear risk there may be violations of international humanitarian law," said Rosa Curling, from Leigh Day.
"The facts on the ground of what is happening in Yemen suggest there is that clear risk."
Ms Curling added: "The current position of the UK government is unlawful.
"They're failing to comply with their own obligations to make sure that they don't export arms when there is a clear risk that those arms may be involved in violations of IHL (International Humanitarian Law)."
The Saudi-led campaign has the backing of the UN Security Council, which gives it legitimacy under international law.
The UK supports the coalition and, along with the US and other countries, has been supplying some of the weapons and jets used in the bombing campaign.
But attacks directed against civilians or civilian targets constitute grave violations of international humanitarian law.
At issue is whether the UK has knowledge that British weapons sold to Saudi Arabia would or could be used to commit such violations, and whether the UK has done enough to satisfy itself that those weapons are not being used to commit war crimes.
That is a requirement under the Arms Trade Treaty, which was championed by Prime Minister David Cameron.
A key piece of evidence, cited by the lawyers, came in an interview on Newsnight with Foreign Secretary Philip Hammond in November.
He said: "The Saudis deny that there have been any breaches of international humanitarian law. Obviously that denial alone is not enough, we need to see proper investigations."
'Degree of scrutiny'
That admission is significant, says Philippe Sands, one of the authors of a separate legal opinion prepared for Amnesty International by the legal firm Matrix Chambers.
"The British government has confirmed that it is aware of the allegations that violations of international law are being committed on the ground in Yemen, and it is also aware that the assurances being given by the Saudi authorities are perhaps not watertight," he said.
"That recognition by the United Kingdom is significant. It heightens the degree of scrutiny which they ought to be applying in the determination of whether any new weapons should be supplied."
Their legal opinion concludes: "On the basis of the evidence available to us, any authorisation by the UK of the transfer of weapons or other items... to Saudi Arabia, in circumstances where such weapons are capable of being used in the conflict in Yemen... would constitute a breach by the UK of its obligations under domestic, European and international law."
The Saudi authorities have consistently denied targeting civilians.
They point out that their campaign comes at the request of Yemen's internationally recognized government, against a rebel force, the Houthis, who themselves stand accused of violating the laws of war. But Britain is not supplying weapons to the Houthis.
A UK government spokesman told Newsnight: "The UK government has stressed, to the Saudi authorities, the importance of conducting transparent investigations into all incidents where it is alleged that international humanitarian law has been breached."
The statement added: "The UK government itself is not carrying out separate investigations into these incidents."