Actor Jamie Waylett, who plays bully Vincent Crabbe in the Harry Potter films, has been arrested on suspicion of possessing cannabis.
A Scotland Yard spokesman said the 19-year-old actor and a friend were arrested on Thursday after police in London stopped the car they were in.
"The car was searched and officers found eight bags of a substance believed to be cannabis," he said.
A raid on Waylett's home uncovered 10 plants, also thought to be cannabis.
"Officers found a number of plants believed to be cannabis plants and equipment for the cultivation," Scotland Yard said.
"Both men have been bailed until July pending further inquiries and tests on the substances."
Film company Warner Brothers have refused to comment about the arrest.
Waylett has appeared in all five Potter films as the right-hand man to Draco Malfoy.
The next instalment - Harry Potter and the Half-Blood Prince - is due for release in July 2009.