Thomson joined the show in 2000 when he was just 14
The actor who plays the son in BBC One sitcom My Family has been cautioned by police after being arrested for possession of Class A drugs.
Gabriel Thomson, 21, who plays Michael Harper in the show, was arrested by police in Battersea in south London.
Officers were called to an address there on Sunday afternoon after receiving reports of an "altercation".
A spokeswoman for the show - which stars Robert Lindsay and Zoe Wanamaker - declined to comment.
A spokesman for the Metropolitan Police said: "A 21-year-old man was arrested just after 3.30pm yesterday afternoon after reports of a disturbance in Kingsley Road, Battersea.
"The 21-year-old was arrested over suspicion of possession of Class A drugs.
"He was taken to a south London police station where he received a caution under advice from the Crown Prosecution Service for possession of a Class A drug."
Thomson's agent, Christian Hodell, said: "We confirm that Gabriel has today received a caution from the police. The caution was for possession of drugs.
"No charges of any nature are going to be brought. The police regard this matter as now closed, and so do we."
Thomson has played geeky Michael Harper in the BBC comedy since 2000.
He also appeared with Jude Law in the film Enemy at the Gates, and played young Pip in a BBC production of Great Expectations.