The leaders of Britain's five main opposition parties have demanded the UK government halts arms sales to Saudi Arabia for use in the war in Yemen, stops all British military operations in the kingdom and condemns Riyadh's conduct in the conflict.
The message was due to be delivered to foreign secretary Jeremy Hunt in a letter marking the fourth anniversary of the civil war between Houthi rebels and the Yemeni government, which is backed by a Saudi-led coalition.
"After four years of intolerable suffering and degradation, time is running out. Yemen stands on the brink of catastrophe," wrote the five leaders, who included Labour's Jeremy Corbyn and the SNP's Ian Blackford, whose party organised the letter.
"The UK government must condemn the reckless and barbaric behaviour of the Saudi government, immediately suspend the sale of arms to the regime for use in the war on Yemen pending a comprehensive and independent investigation of all alleged war crimes, and halt all conventional military operations in the country."
The leaders, also including Sir Vince Cable of the Liberal Democrats, Liz Saville Roberts of Plaid Cymru and the Green Party's Caroline Lucas, said they welcomed a visit by Mr Hunt to Yemen at the start of the month.
He became the first British foreign secretary to visit Yemen in more than 20 years.
But they raised allegations made by colleagues last October of the UK being complicit "in fuelling this conflict for profit" and listed what they described as crimes by the Saudi-led coalition.
This included 10,852 of the 17,640 UN documented civilian casualties in Yemen.
"There are 20 million people who don't know if they're going to be able to get their next week's supply of food, and 85,000 children are estimated to have died of starvation in the country," the opposition leaders said.
They accused the UK of being unwilling to accept any correlation between the UK's arms trade and support for the Saudi government, a key ally in the region.
The leaders said the UK's £4.6bn of arms export licences to Saudi Arabia "eclipses" the aid given to the people of Yemen.
"You and your colleagues in government have often affirmed that the UK's closeness to Saudi Arabia gives the UK influence over them," the letter said.
"Given the extent to which we supply weapons to Saudi Arabia, the leverage we hold over them must be considerable yet their behaviours and appalling human rights abuses have continued unrestrained," it said.
"The UK cannot be a partner for peace if it continues to sell arms to a murderous regime with one hand and give aid - miniscule in comparison - with the other."