Harry Potter and the Philosopher's Stone DVD. (Handout)

Actor Nicholas  Read, who played a goblin in Harry Potter and the Philosopher's Stone,  escaped a jail sentence for exposing himself on a train after a judge ruled he would be "a very vulnerable person" in prison.

The 41-year-old dwarf, who has also appeared as an Ewok in the Star Wars: Episode VI - Return of the Jedi, flashed at a student on a train to Manchester, north England and took a picture of her horrified friend on a cell phone.

The offence put him in breach of a 20-week suspended jail sentence which was handed to him after he performed a sex act in front of a teenager on a train journey in 2010, and a judge warned in June he faced imprisonment when he was sentenced.

Read returned to Manchester Crown Court on Friday, and Judge Roger Thomas QC handed him a suspended 16-week jail term. Thomas told the actor he would be "a very vulnerable person in the prison system". He also ordered Read to sign the sex offenders' register for seven years and urged him to carry on with his sex offenders' treatment program.

The judge added, "You have been very frank and straightforward and in the past often denied what you have done. You are now finally facing up to the difficulties you face. I'm not going to disrupt that progress by sending you to prison for only a matter of weeks. It would be better to see if the progress you are making can be contained.

"The courts will have to send you to prison if you commit another offence. That is inevitable and nothing can save you."