Cliff Richard police probe 'expanding'

British police say the investigation into Sir Cliff Richard has increased significantly in size and involves more than one allegation.

Police continue their investigation into alleged sexual assaults by Sir Cliff Richard. (AAP)

South Yorkshire's Chief Constable David Crompton says the expanding nature of the probe into an historical sex offence means he cannot give a date when it will be concluded.

In a letter to Keith Vaz, chairman of the Commons Home Affairs Select Committee, Crompton said the force was in regular contact with Sir Cliff's lawyers.

The publication of the letter from Chief Cnst Crompton, dated February 10, followed a scathing independent report into an agreement between South Yorkshire Police and the BBC, which led to a raid on the British singer's home being broadcast live around the world.

Following the raid, Sir Cliff was interviewed by detectives investigating a claim of a sex crime involving a young boy, but was not arrested or charged.