Senior Tory MPs have criticised the UK's handling of arms sales to Saudi Arabia after the court of appeal ruled on Thursday that they are unlawful.
Julian Lewis, chair of the Commons defence committee, said Britain's relationship with the Gulf state had become "harder and harder to justify".
And former Cabinet minister Andrew Mitchell said the UK must stop marking its "own homework" and allow the United Nations to investigate potential breaches of international humanitarian law.
It came as the United States Senate voted to block arms sales to Saudi Arabia. President Donald Trump is expected to veto the legislation.
UK international trade secretary Liam Fox told the House of Commons that the government would appeal the ruling, prompting fury from opposition politicians.
In his judgment, master of the rolls Sir Terence Etherton found that the government had "made no concluded assessments of whether the Saudi-led coalition had committed violations of international humanitarian law in the past, during the Yemen conflict, and made no attempt to do so".
It follows tens of thousands of civilian deaths in war-torn Yemen in recent years, with many attributed to Saudi-led coalition airstrikes — including the deaths of 40 children in a school bus in August.
The court ruling, following a challenge brought by Campaign Against Arms Trade, ordered a review of arms sales to Saudi Arabia but did not call for their immediate suspension.
Fox said the government would not grant any new export licences for arms to Saudi Arabia while it considers the judgment.
In the Commons, Lewis warned Fox: "As the years have rolled by since the 9/11 atrocities, it has become harder and harder to justify the closeness of our relationship with Saudi Arabia."
Mitchell said: "I have the most profound reservations about the government policy in respect of what is happening in Yemen today, but I have never called for an arms embargo for the simple reason that, in terms of the humanitarian effect, I think it will have very little impact on that.
"I would say to the government front bench that they should listen more carefully to what Parliament has said consistently in almost every debate on this matter over the last three years. ... The argument has been that in all these investigations of breaches of international humanitarian law, it is wrong for Britain, on one side of the conflict, to mark their own homework.
"It is essential that these breaches are looked at internationally by an accepted and impartial force like the UN. And if the government had heeded those warnings from the House of Commons, they would not be in the position they are in today."
The Campaign Against Arms Trade (CAAT) said the UK had licensed £4.7 billion worth of arms, including aircraft, drones, and missiles, to Saudi Arabia since 2015.
The UK government has long been accused of being complicit in attacks on civilians in Yemen by selling Saudi Arabia billions of pounds’ worth of arms and embedding UK military liaison officers and advisers in the country.
But UK ministers have insisted that Britain has no role in executing air strikes or in setting the policy of the Saudi-led coalition.
It is unclear what the next prime minister's stance on Saudi Arabia will be. BuzzFeed News contacted the candidates in the Tory leadership contest for comment.
Boris Johnson and Jeremy Hunt declined to comment, and Michael Gove had not replied at the time of writing.
Andrew Smith of CAAT told BuzzFeed News: "It's no surprise that the government is going to try to appeal this ruling.
"It has done everything it can to continue arming and supporting the brutal Saudi regime. It has always cared more about arms exports than human rights.
"The case we put forward was a very strong one, and we are confident that the government will lose again. It's time for it to stop the arms sales and end its complicity in this terrible war."
SNP spokesperson Stewart Hosie urged the government to rethink its decision to appeal, saying that it had already spent over £100,000 in legal costs and was "throwing good money after bad".
Labour's Barry Gardiner said the decision to appeal was "shameful" and demanded an independent investigation into the government's handling of arms sales to Saudi Arabia.
"We are supposed to be the guardians of international humanitarian law, not the people found in breach of it," he said.
Yasmine Ahmed from campaign group Rights Watch UK said: "This judgment is a victory for the innocent civilians of Yemen who are being subjected to unimaginable horrors."
Lucy Claridge from Amnesty International said: "This is the first time that a UK court has acknowledged the risks of continuing to lavish Saudi Arabia with military equipment for use in Yemen. We welcome this judgment as a major step towards preventing further bloodshed."
And Lib Dem foreign affairs spokesperson Jo Swinson said: "Saudi Arabia is an enemy of British values, including human rights and the rule of law. Their repeated violation and disregard for human rights should have ruled them out as an arms trading partner long ago.
"Instead the Conservative government have continued to export arms and equipment to this brutal regime. The situation is inexcusable and cannot continue."
A government spokesperson said: "This judgment is not about whether the decisions themselves were right or wrong, but whether the process in reaching those decisions was correct. We disagree with the judgment and will be seeking permission to appeal."