BAE Systems has moved a step closer to sealing a multi-billion pound contract to supply Typhoon fighter jets to Saudi Arabia.
On the last day of Crown Prince Mohammed bin Salman's three-day visit to Britain, the UK and Saudi governments have signed a memorandum of intent to finalise discussions on the deal.
In a statement, BAE Systems described it as a "positive step towards agreeing a contract" for the 48 state-of-the-art planes.
The Crown Prince, 32, met Defence Secretary Gavin Williamson at RAF Northolt ahead of his return to the desert kingdom.
His visit also included talks with Prime Minister Theresa May and the Archbishop of Canterbury, as well as an audience with the Queen, and saw agreement on an estimated £65 billion worth of additional trade in the coming years as part of the Crown Prince's Vision 2030 economic plan.
Any order for UK-built warplanes would spark protests from anti-war campaigners, who mounted demonstrations during the Crown Prince's visit over Saudi's military intervention in Yemen.
Andrew Smith of Campaign Against Arms Trade (CAAT) said the UK had already licensed £4.6 billion of arms for Saudi Arabia since hostilities began in Yemen in 2015.
"If agreed, this shameful deal will be celebrated in the palaces of Riyadh and by the arms companies who will profit from it, but it will mean even greater destruction for the people of Yemen," said Mr Smith.
"If May really cares for the rights of people being repressed in Saudi Arabia, or bombed in Yemen, then she must stop arming and supporting the brutal Saudi dictatorship."
A BAE Systems spokesman said: "The UK Government has signed a Memorandum of Intent with the Kingdom of Saudi Arabia to aim to finalise discussions for the purchase of 48 Typhoon Aircraft.
"This is a positive step towards agreeing a contract for our valued partner. We are committed to supporting the Kingdom as it modernises the Saudi armed forces and develops key industrial capabilities critical to the delivery of Vision 2030."
Save the Children's chief executive Kevin Watkins warned Saudi Arabia is targeting children in the brutal war in Yemen with a "growing sense of impunity".
Mr Watkins criticised the welcome given to the crown prince, saying that in Yemen the Saudis were "orchestrating what will potentially become the worst famine in the last 50 years".
Speaking in London at the launch of The War On Children, a report into crimes against children in warzones, Mr Watkins said: "It has become acceptable to operate humanitarian blockades which, if not explicitly designed to starve children and harm children, will have that inevitable consequence.
"The fact that we have the head of state of a government that has been operating such a blockade - Saudi Arabia - recently invited to Buckingham Palace and Downing Street while the military... is orchestrating what will potentially become the worst famine in the last 50 years, I think speaks volumes to another aspect of the problem that I want to highlight - the growing sense of impunity surrounding these crimes against children.
"The fact that you can rape, murder, kidnap, bomb schools, bomb clinics with no consequence, speaks I think to the heart of the deeper challenge that we are addressing today."
Yemen, which borders southern Saudi Arabia, has been embroiled in a bloody civil war since 2014 when rebels took over the capital city of Sana'a.
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.
Prince Mohammed is the son of the 82-year-old King Salman and his visit comes at a time when he is taking steps to modernise the ultra-conservative Arab state.
Ahead of his visit, the crown prince signalled the importance of the UK-Saudi Arabia relationship to security, saying both countries would be "much safer if you have a strong relationship with Saudi Arabia".
The crown prince was treated to a private dinner with the Prime Minister at Chequers on Thursday night and has also met with the Prince of Wales and Duke of Cambridge.
Regarding Mrs May's meeting, a Downing Street spokesman said: "The Prime Minister raised our deep concerns at the humanitarian situation in Yemen.
"The Prime Minister and Crown Prince agreed on the importance of full and unfettered humanitarian and commercial access, including through the ports, and that a political solution was ultimately the only way to end the conflict and humanitarian suffering in Yemen."