A teenager has been put on the Sex Offenders' Register for sexually assaulting a girl at Harry Potter star Rupert Grint's 18th birthday party.

Joshua Lodge, 18, one of the red-haired actor's school friends, was caught in an outbuilding with the underage teen.

The man who discovered the pair ordered Lodge off the premises - but police later launched an full investigation.

The incident happened at a lavish party for 120 guests at an address in Watton-at-Stone, Herts on September 9 and 10 last year.

At one stage, it was anticipated Rupert, who plays Ron Weasley, in the blockbuster films, would be called to give evidence.

But prosecutors decided against pursuing the case to trial on the basis that both the girl, who cannot be named for legal reasons, and the defendant had had too much to drink.

St Albans Crown Court heard that lawyers now accepted Lodge, who is of previously good character, should be given a caution.

Prosecutor Ann Evans said: "I take the view that the nature of the offence was such that two teenagers at a private party where unfortunately, on all sides, too much drink was taken a caution would be appropriate.

"After considering the matter over the last few days and, having taken the opportunity to speak to the victim and her family, the crown propose that the defendant is cautioned for this offence.

"The defence is prepared to accede to this. I am sorry this case has been listed for trial and we have not been able to reach a decision until today."

Although the 19-year-old denied a charge of sexual activity with a child, the caution means his name will now appear on the Sex Offenders' Register.

Judge Marie Catterson was told that Lodge, of Ruskin Close, Cheshunt, Herts was cautioned at St Albans police station.

The case is due back in court to be closed later.