Sir Cliff Richard is unsure who is accusing him of sexual assault.

The 73-year-old singer is being investigated over an historic allegation that he sexually assaulted a schoolboy at a religious rally held in Sheffield, England in 1985.

The Sunday Mirror reports that Cliff told a friend: ''I've no idea who's behind this. It's so unfair.''

A friend of the 'Summer Holiday' hitmaker added: ''He's mystified by all of this. He doesn't know his accuser. He has no idea when his ordeal will end.

''Sir Cliff very much wants to return to performing full-time without a stain on his character. This has affected him greatly, and he just wants to get on with his life.''

Detectives searched the star's ­apartment in Sunningdale, Berkshire last month over the accusations.

Earlier this week, the star took to his Facebook page to deny the allegations and thank his fans for their support.

The post read: ''Hi everyone, this is just a brief message to thank all of you who have shown support, including those in the Press.

''I wanted to let you know that I'm doing ok and it goes without saying that I fully intend to clear my name.

''Just to be absolutely clear, the allegation is completely false and I will continue to cooperate with the police should they wish to speak to me again.

''Once again ... many thanks! (sic)''

Cliff is yet to be formally arrested or charged for the offence.