Theresa May has confronted the Saudi crown prince over the murder of Jamal Khashoggi.
The Prime Minister held a meeting with Mohammed bin Salman at the G20 summit in Buenos Aires, Argentina, on Friday.
Mrs May told the crown prince to "co-operate fully" with the Turkish authorities as they investigate the murder.
She also demanded Saudi Arabia "takes action" to ensure "such a deplorable incident could not happen again".
It follows French president Emmanuel Macron's extraordinary confrontation with the crown prince, captured on film, in which he said: "I am worried. You never listen to me."
Journalist Mr Khashoggi, a critic of the crown prince's regime, was killed at the Saudi consulate in Istanbul on October 2.
After Mrs May's meeting, a Downing Street spokesman said: "The Prime Minister stressed the importance of ensuring that those responsible for the appalling murder of Jamal Khashoggi are held to account, and that Saudi Arabia takes action to build confidence that such a deplorable incident could not happen again.
"Noting the steps taken by the Saudi investigation since the foreign secretary had met with the crown prince and King Salman on 12 November, she encouraged the crown prince to ensure that Saudi Arabia co-operated fully with the Turkish authorities and worked to bring both investigations to an acceptable close.
"To ensure full accountability, there needed to be full transparency about exactly what had happened and who was responsible, in line with the commitments made by King Salman when she spoke to him on October 24."
Mrs May had been forced to defend the meeting earlier on Friday, stressing she would tell him there needed to be a “full and transparent investigation” into the murder.
The crown prince has appeared isolated among fellow world leaders at the G20 summit, with the exception of a vigorous public handshake from Russia's Vladimir Putin - who himself has been treated as a pariah by some of those present.
