Jeremy Corbyn has demanded the UK end its policy of selling £4.5billion of "deadly weapons" to Saudi Arabia in an angry PMQs attack.
Labour's leader told Theresa May "save those lives!" as he blasted her for not going far enough after a Court of Appeal defeat.
Top judges ruled last week the UK
Britain has now suspended new arms exports licences to the country until the case is resolved. But the UK government wants to appeal.
Mr Corbyn said instead of appealing, the judgement should be a "wake up call".
"There is overwhelming evidence that war crimes are being committed in Yemen by Saudi forces," he said.
"Instead of appealing the judgement why not accept the judgement and stop arms sales to Saudi Arabia now and bring about peace in the Yemen and save those lives."
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.
Mr Corbyn said: "There is overwhelming evidence that war crimes are being committed in Yemen by Saudi Arabian forces.
"A [regime] that flouts every human rights law at home and abroad.
"Its government believes it can kill with impunity journalists and civil rights campaigners, Yemenis or Bahranis.
"It funds extremism around the world.
"But the UK has supplied them with over £4.5bnillion of deadly weapons.
"UK weapons which have been used in indiscriminate attacks on civilians in which over 200,000 people have been killed.
"Hundreds of thousands more stand on the brink of famine, starvation and a death from wholly preventable diseases."
Mrs May furiously hit back, branding Labour "on the wrong side of the argument" after an MP invited rebel Houthi leaders into the Commons.
Lloyd Russell-Moyle scrapped the invite after the man's anti-Semitic comments came to light.
Mrs May said Britain takes arms exports licences “very seriously” and "we are all concerned about the humanitarian situation in Yemen."
She added finding peace was exactly what Britain was trying to do, and co-operation with Saudis had saved lives from terrorism.
She said: "We’re disappointed the court found against the government on one ground and we will be seeking permission to appeal this judgement."
Yet even Conservative MPs clashed over the policy.
Former Foreign Office minister Alistair Burt said Houthi forces were themselves accused of war crimes and Mr Corbyn was being "one sided".
"It's best to recognise the horrors of war on all sides," he said.
But another former minister, Andrew Mitchell, took the opposite tack.
The Tory MP urged Mrs May to condemn "night after night" bombings by the Saudis that are killing innocent civilians and "radicalising" young Yemenis.
