An actor in a hit BBC comedy has been given a caution for possessing cocaine - just days after Sir Ian Blair said celebrities caught on camera apparently taking drugs should be prosecuted.

Police decided that Gabriel Thomson should be spared court, despite the warning from the Metropolitan Police Commissioner that he wanted to see juries, not prosecutors, deciding on whether celebrities are guilty of taking drugs if footage of them with what appears to be illegal substances exists.

Public figures, including Amy Winehouse and Kate Moss, have escaped prosecution despite the publication of video footage which appeared to show them taking drugs. Sir Ian wants the Crown Prosecution Service to look again at current rules which state police cannot charge someone if they are unsure what substance they have been filmed taking.

The actor, star of BBC 1 comedy My Family, was led in handcuffs from his family's home in Battersea when he was arrested on Sunday afternoon. A Scotland Yard spokesman said: "A 21-year-old man has been cautioned for possession of a Class A substance."

Thomson, who plays Michael Harper in the sitcom, was arrested after officers found substances thought to be cocaine and cannabis in his possession. It comes as former Europe minister Denis MacShane raised the issue in the Commons of celebrity "role models".

He urged ministers to support Sir Ian's views on "celebrity cocaine users and deplore the rather odd jobsworth response of the Director of Public Prosecutions [Sir Ken Macdonald] saying it's all very difficult to deal with these celebrities".

He added: "Internationally if you want to reduce supply you have to reduce demand. The Metropolitan Commissioner is right and the DPP is wrong."

Junior Home Office minister Vernon Coaker said: "Of course people who act as role models for young people and others have to accept the responsibility they have. Class A drug use overall has remained stable. The problem we've had with Class A drugs is actually the use of cocaine - powder cocaine."

Mr Coaker said that a campaign had been launched with the Colombian president, which showed the impact cocaine had on Colombia and "the way it wrecks that country and prevents them moving forward". "Perhaps those who use cocaine should reflect on the impact in Colombia as well as in their own communities," he said.

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