A teen who threw a chain off a motorway bridge which hit a passing van has been sentenced to 14 months in a young offenders' institution.

Joshua Millar, an 18-year-old American citizen from Cobham, pleaded guilty to criminal damage and being reckless as to whether life would be endangered. He was sentenced at Kingston Crown Court yesterday.

Sentencing, Judge Binning said: "In my mind only a custodial sentence is appropriate.

"It was very fortunate you did not cause severe injuries or death."

Andrew Stephens, prosecuting, told the court Millar was out with friends in Elmbridge on the night of January 10, when he saw a heavy link chain and decided to pick it up.

After driving around Esher and Hersham, they stopped on Cobham bridge between junctions 8 and 9 of the M25 at around 4am.

Millar decided to throw the chain off the bridge as he did not want to take it home with him.

The chain struck a passing white van, smashing the windscreen and injuring the driver, who was forced to pull on to the hard shoulder.

Forensics tests on the chain recovered DNA evidence and the results led police to arrest Millar two months later.

When initially questioned by police about why he threw the chain, Millar responded that he did not know.

He added: "I was just bored and causing trouble, I don't know, I was just bored."

He later confessed he thought it might be fun to watch a car's tyre pop and that it would be more exciting to throw the chain off a motorway bridge than just into the verge.

The court heard how the driver of the van, Mr Bush, was still traumatised by the incident and often got colleagues to drive on the M25 as he was nervous going under bridges.

Defence counsel, George Zachary, said Millar was a promising actor who had recently gained a scholarship at an acting school in Florida.

He added: "This was a very stupid act but not a malicious one. These proceedings have been a massive wake up call to him."