Detectives investigating the disappearance of teenager Lee Boxell 26 years ago have announced a £20,000 reward for information.

Police fear that Lee, who went missing at the age of 15 in 1988, was killed after either witnessing child abuse or being the victim of sexual assault at a popular teenage hang-out in Cheam, south-west London.

The launch of the reward, for information leading to the recovery of Lee's body and a subsequent prosecution, comes in the same week Lee would have celebrated his 41st birthday.

A review of the disappearance suggests that Lee used to attend an unofficial youth club in an out-building at St Dunstan's Church in Church Lane, which was known locally as the "shed".

It was a place where youngsters could gather to drink and smoke. It is possible that he went to the shed to meet friends the day he went missing, Saturday September 10.

Officers also know of paedophile activity amongst a group of offenders in the Cheam area at the time Lee disappeared.

Detective Inspector John McQuade, the investigating officer, said: "We have been trying to solve the mystery of Lee's disappearance for 26 years now and have carried out extensive enquiries and appeals over the years.

"There are two working theories - that Lee was the victim of a sexual assault that resulted in his death or that he intervened to try to help someone else and was then assaulted.

"We hope the offer of this reward may encourage people who have not previously felt able to come forward to now do so. Someone knows what happened to Lee and where his body is buried.

"We are still appealing for any victim of sexual assault around that time, linked to the area or the shed, to please contact us in confidence. They could hold information vital to our investigation that would help us piece together Lee's last movements."

In the Metropolitan Police Service's longest ever forensic archaeological dig, police spent a year excavating St Dunstan's churchyard. Nothing was found but inquiries continue.

Officers are also keen to speak to anyone who attended the "shed" in or around 1988. A number of initials have been carved into the church wall and detectives are keen for people to think back and contact them with any information.

Peter Boxell, Lee's father, said: "I am pleased the police are funding this reward and would appeal to anyone with information to come forward.

"We, Lee's family, need answers to what happened to Lee. Someone in this community can give us those answers. Please help us bring Lee home."

Mr Boxell has today also released a recording of a song called Where Is Lee? that he sang at the Missing People Carol Service, hosted by Sir Trevor McDonald, in December.

Since renewing public appeals, including an appearance on Crimewatch last year, police have received an allegation of sexual abuse that had never been reported before.

The teenager was last seen in the high street in Sutton in September 1988, and detectives originally assumed he had gone to watch a football match at Selhurst Park.

No trace of him has been found or any indication that he is still alive, such as using a bank.

When he disappeared, Lee was 5ft 6in, slim with light brown hair and was wearing black jeans, a white Flintstones T-shirt and brown suede shoes.

Anyone with information can call Sutton police on 020 8721 4005 or Crimestoppers anonymously on 0800 555111.