Saudi Arabia's crown prince is starting a three-day visit to the UK later amid protests planned against his country's role in the war in Yemen.
Mohammed bin Salman, 32, is seen by some as a modernising force in the Gulf State.
He is due to hold talks with Theresa May and have lunch with the Queen.
The UK hopes to capitalise on the Saudi economy's opening-up but No 10 said the PM would also express "deep concern" at the humanitarian situation in Yemen.
Demonstrators protesting against the Saudi-led coalition bombing of rebel Houthi forces in Yemen are expected outside Downing Street – where Mohammed bin Salman will meet Mrs May and other cabinet ministers.
The crown prince, who is regarded as being heir presumptive to the 82-year old King Salman, is making his first visit to the UK since taking up the role last year.
He will also have dinner with the Prince of Wales and the Duke of Cambridge.
He is credited with kick-starting domestic reforms in the ultra-conservative desert kingdom, such as the upcoming lifting of the ban on women driving and the re-opening of commercial cinemas after 35 years, as well as tackling corruption within the government and ruling Al Saud family.
Media playback is unsupported on your device
A new Strategic Partnership Council will be established which it is hoped could lead to Saudi investment in and through the UK of up to £100bn during the next 10 years.
'Brutal and illegal'
The conflict between the Saudi-backed government in Yemen and the Houthis, backed by Iran, is seen by many as part of a regional power struggle between the two countries.
According to the United Nations, more than 9,000 people, over half of them civilians, have been killed and more than 52,000 injured in fighting in Yemen since March 2015.
Downing Street said Mrs May will "acknowledge the steps" taken recently by Saudi Arabia to address the crisis but will stress the importance of "full and unfettered humanitarian and commercial access" through ports which have previously been hit by coalition blockades.
The PM will also call for urgent progress on securing a political resolution to the crisis, her spokesman said.
Anti-war campaigners, including the Labour leader Jeremy Corbyn, have accused the government, a leading exporter of arms to the Saudi capital Riyadh, of being "complicit" in coalition bombings in Yemen.
[contf] [contfnew] 
BBC
[contfnewc] [contfnewc]
