The UK's policy of selling weapons to Saudi Arabia has been dealt a landmark defeat in the Court of Appeal.
Tory ministers and Boris Johnson are accused of presiding over a "national shame" after campaigners won a legal challenge against the UK government.
Shamefaced ministers temporarily stopped all new arms export licences to the Gulf state following today's ruling.
Yet they vowed to APPEAL the judgement - which could force a Supreme Court fight and new sales in future.
In today's ruling, three top judges said the UK made no attempt to properly assess if Saudi Arabia had breached international humanitarian law in Yemen.
That meant the government acted in an "irrational and unlawful" way by allowing weapon sales to the nation.
However, the judges said their ruling did not mean arms exports to Saudi Arabia must immediately be suspended.
Instead, Sir Terence Etherton, Master of the Rolls, said the Government must "reconsider" and estimate any future risks in light of the court's conclusions.
Even so today's decision is a savage indictment of Theresa May's refusal to halt weapons sales to the country.
And it is damning for the likely next PM Boris Johnson. According to the Guardian, as Foreign Secretary he approved the sale of bomb parts to Saudi Arabia in 2016 -days after an airstrike at a factory killed 14.
It covers decisions made between December 2015 and February 2017, which includes Mr Johnson's time in office.
International Trade Secretary Liam Fox told MPs: "We disagree with the judgement and will seek permission to appeal.
"Alongside this we are carefully considering the implications of the judgement for decision making.
"While we do this we will not grant any new licences for export to Saudi Arabia and its coalition partners which might be used in the conflict in Yemen."
Mr Fox claimed the ruling was a criticism of the UK's processes - not arms exports decisions themselves. And he said Saudi Arabia were an important ally in the fight against terror.
But Shadow Trade Secretary Barry Gardiner said the judgement was a "damning indictment" and a "national shame".
“We are supposed to be the guardians of international humanitarian law - not the people found in breach of it," he said. Mr Fox said that claim was "outrageous" and demanded Labour retracted it.
Mr Gardiner welcomed the suspension of new arms export licences but said it was "not enough".
He demanded an independent investigation and branded the decision to appeal the judgement "shameful".
And the decision prompted furious calls for the UK to fully suspend weapons sales to the Gulf state.
Andrew Smith of Campaign Against Arms Trade, which brought the challenge, said arms sales must now "stop immediately".
He added: "We welcome this verdict, but it should never have taken a court case brought by campaigners to force the Government to follow its own rules.
“The bombing has created the worst humanitarian crisis in the world. UK arms companies have profited every step of the way."
Leigh Day solicitor Rosa Curling, who represented the campaigners, added: "The horrors that the world has witnessed in Yemen can no longer be ignored by the UK government."
Lib Dem foreign affairs spokeswoman and leadership candidate 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."
Labour Leader Jeremy Corbyn said: "The UK government should accept today’s court ruling that arms sales to Saudi Arabia are unlawful because of their use against civilians in Yemen.
"UK advice, assistance and arms supplies to Saudi's war in Yemen is a moral stain on our country. Arms sales to Saudi must stop now."
Emily Thornberry, Labour's Shadow Foreign Secretary, said: "This devastating judgement proves everything Labour has been saying for years: that Ministers have wilfully disregarded the evidence that Saudi Arabia was violating international humanitarian law in Yemen, while nevertheless continuing to supply them with weapons.
“What we now need is a full parliamentary or public inquiry to find out how that was allowed to happen, and which Ministers were responsible for those breaches of the law."
Lucy Claridge, Amnesty International’s Director of Strategic Litigation, said: "We welcome this judgment as a major step towards preventing further bloodshed."
International Trade Secretary Liam Fox was due to give a statement to the House of Commons at 12.30pm on how the government planned to respond to the judgement.
Yemen, which borders southern Saudi Arabia, has been embroiled in a bloody civil war since Houthi rebels took over the capital city of Sana'a in early 2015.
Saudi Arabia has led a coalition supporting the Yemeni government against the Houthis in a war which has caused a humanitarian and civilian catastrophe.
Yet Britain has refused to stop selling weapons to Saudi Arabia - a key strategic ally to the West - despite an outcry over the deaths in Yemen.
And sales even continued after journalist Jamal Khashoggi's brutal murder in the Saudi consulate in Istanbul in October 2018.
The High Court ruled in July 2017 that the export of weapons was lawful. But the Court of Appeal today overturned that decision.
In its written ruling, the Court of Appeal stated: "The question whether there was an historic pattern of breaches of international humanitarian law on the part of the (Saudi-led) coalition, and Saudi Arabia in particular, was a question which required to be faced.
"Even if it could not be answered with reasonable confidence in respect of every incident of concern ... it is clear to us that it could properly be answered in respect of many such incidents, including most, if not all, of those which have featured prominently in argument. At least the attempt had to be made."
A government spokesman said: "This judgement 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."