Video report by ITV News International Affairs Editor, Rageh Omaar
Jeremy Corbyn and Theresa May have had a heated exchange over Saudi Arabia's role in the Yemen civil war, with the Labour leader accusing the Government of "colluding" in war crimes by Saudi forces.
As Saudi Crown Prince Mohammed bin Salman began his three-day visit to the UK, Mr Corbyn said British military personnel were "directing" Saudi military operations responsible for large-scale civilian casualties.
He called on Theresa May to stop British arms sales to Saudi Arabia and to demand an immediate ceasefire in Yemen.
A spokesman for Mr Corbyn said he was referring to British military advisors helping the Saudi air force targeting Houthi rebels, and that their role to prevent civilian injuries and deaths had been a "disastrous failure".
In the Commons, Mrs May strongly defended the UK's "historic" ties with the Saudis, saying they had "saved the lives of potentially hundreds of people in this country" - a reference to UK-Saudi counter-terror co-operations.
However Mr Corbyn said a "humanitarian disaster" was taking place in Yemen as a result of the Saudi-led aerial bombing campaign and its blockade of Yemeni ports.
"Germany has suspended arms sales to Saudi Arabia but British arms sales have sharply increased and British military advisers are directing the war," he said.
"It cannot be right that her Government is colluding in what the United Nations says is evidence of war crimes."
Mrs May's official spokesman said: "The suggestion that British military advisers are directing the war is simply not true.
"The UK is not a member of the Saudi-led coalition operating in Yemen. British personnel are not involved in carrying out strikes, directing or conducting operations in Yemen and are not involved in the Saudi targeting and decision-making process.
"We have a long-standing relationship with Saudi Arabia in terms of supporting Saudi compliance with international humanitarian law and in armed forces training."
Mrs May is currently meeting with the crown prince, where she will raise "deep concerns" over the Yemen crisis.
Earlier the crown prince was received by the Queen at Buckingham Palace at the start of a trip that is set to be dominated by protests over his country's role in the bloody civil war.
The Prime Minister will call for urgent progress on securing a political resolution to the crisis when she meets the crown prince.
"You can expect them to discuss Yemen and the Prime Minister to raise deep concerns at the humanitarian situation," the PM's official spokesman said.
Yemen has been embroiled in the civil war since 2014 when rebels took over the capital city of Sanaa.
Saudi Arabia is the main player in a coalition supporting the Yemeni government against the Houthis in a war which has caused a humanitarian catastrophe..
The prince was given a warn welcome by the British monarch today.
He met the Queen in the private audience room at Buckingham Palace, ahead of a lunch at the London residence. The Duke of York was also present for luncheon, the palace said.
But he faces questions from the PM, whose spokesman said she will "acknowledge the steps" taken recently by Saudi Arabia to address the crisis but will push for more.
She will "stress the importance of full and unfettered humanitarian and commercial access" through the ports of Hodeidah and Salif, which have previously been hit by coalition blockades.
"She will also reiterate how seriously we take allegations of violations against international humanitarian law and emphasise the need to ensure that these are investigated swiftly and thoroughly," the spokesman added.
"She will make clear that we urgently need to see progress on the political track, which is ultimately the only way to end the conflict and humanitarian suffering in Yemen."
The crown prince has been the driving force behind a modernisation programme, Vision 2030, in Saudi Arabia, but the reforms have been dismissed as a "mirage" by campaigners.
The inaugural meeting of a UK Saudi strategic partnership council , which will meet annually to discuss bilateral and international issues, will be held at No 10 and attended by UK and Saudi ministers.
It is hoped the forum could lead to Saudi investment in and through the UK of up to £100 billion over the next 10 years.
The crown prince has also been granted rare access to a briefing on foreign policy issues, including Yemen, by national security officials.
On Thursday the crown prince will head to Chequers for talks and a private dinner with the PM that will focus on foreign policy issues, including Yemen and Iran.
On Friday, he will meet Defence Secretary Gavin Williamson for talks.
The Government has faced criticism over its arms sales to the kingdom but Downing Street insists it "operates one of the most robust arms export control regimes in the world".
Human rights campaigners are planning to stage a protest outside Downing Street at 5pm on Wednesday.
According to analysis by human rights charity Reprieve, executions have doubled under the Crown Prince.
The organisation said since his appointment in July 2017, 133 people had been executed compared with 67 in the previous eight months.