The inquiry began last August after police raided the star's Sunningdale home, and South Yorkshire Police have reportedly said the inquiry has 'increased significantly' and now involves more than one allegation against Sir Cliff.
However the singer says he has never assaulted anyone and is fully cooperating with the police.
The BBC reported South Yorkshire Police's chief constable as saying there was no date for when their investigation would conclude.
Sir Cliff was interviewed last year about a claim of a sex crime involving a young boy, but was not arrested or charged.
The South Yorkshire Police communications team refused to comment further on the case when approached this evening.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article