"People who have long thought of themselves as Saudi's friends are saying this is a very, very serious matter," UK foreign secretary Jeremy Hunt told the AFP. "We are extremely worried."
The British foreign secretary also said in a statement that if the allegations were true "that would be extremely concerning and the UK will treat the incident very seriously," adding that "friendships depend on shared values."
Hunt said he had spoken to Saudi Foreign Minister Adel al-Jubeir on Tuesday to tell him "how very, very concerned the United Kingdom is".
Jamal Khashoggi, a contributor to the Washington Post, has been missing since October 2 after allegedly visiting the Saudi consulate in Istanbul to acquire documentation for his upcoming marriage.
Turkish officials have demanded Saudi authorities to release video of Khashoggi's activity at the consulate and alleged that the journalist could have died.
Riyadh strongly denied all allegations of any involvement in the case on Tuesday in a statement issued by Saudi ambassador to the US Prince Khalid bin Salman bin Abdulaziz.
The statement called the claims "outrageous" and warned of "malicious leaks and grim rumors" regarding the circumstances of Khashoggi's disappearance and hoped to clarify matters through the ongoing investigation into his whereabouts.
"Jamal has many friends in the Kingdom, including myself, and despite our differences, and his choice to go into his so called ‘self-exile,' we still maintained regular contact when he was in Washington," Prince Khalid said.
The statement also denounced Turkish accusations as "absolutely false, and baseless", adding that reports had changed multiple times and that Turkish officials and the media were allowed to inspect the consulate. "I don't know who is behind these claims, or their intentions, nor do I care frankly," the statement read.
"What we do care about is Jamal's wellbeing," the statement continued, adding that "The Saudi Consulate is fully cooperating with the local authorities to uncover what happened after he left" by dispatching a security team with the Turkish government's approval for the investigation.