The Sun on Sunday reported that the alleged cocaine use by the stars - who have not been named - was widely talked about on the show. BBC News understands the corporation has hired a law firm to help it carry out the probe. The BBC said  that it had "clear protocols and policies in place" for dealing with any serious complaints raised with it.

According to the Sun on Sunday, the claims of alleged drug use were made in a legal submission to the BBC in March by law firm Russells on behalf of Wynne Evans, a former celebrity contestant on the show. Evans has been approached for a comment by BBC News. The newspaper claims others had also reported allegations of drug taking on the show to the BBC. In a statement on Saturday night, a BBC spokesperson added: "We would always encourage people to speak to us if they have concerns. "It would not be appropriate for us to comment further."

It is understood it is not unusual for the corporation to appoint external law firms to help it deliver BBC-led investigations. In these cases, they would report back to an internal team. Strictly, which has been airing since 2004, has faced multiple controversies over the past year relating to the behaviour of some of its professionals and celebrity guests. Professional dancers Giovanni Pernice and Graziano Di Prima left the show last year following allegations about their behaviour towards their dance partners.

The BBC announced new welfare measures for Strictly last July. These include having chaperones in all rehearsal rooms, adding two new welfare producers and providing additional training for the professional dancers, production team and crew. In January, Welsh opera singer Evans, who had been a celebrity dancer in last year's series, made what he described as an "inappropriate and unacceptable" comment during the Strictly live tour launch. He took time off from his BBC Radio Wales daytime show after the incident, and the BBC said in May that he will not be returning to it. (BBC)