Workmen broke into a sealed room and found a knife that could unlock a 35-year-old murder mystery.

They called a locksmith to enter the room in the bowels of Brighton police station in John Street when no one could come up with a key.

Inside was a treasure trove of evidence lost to police for decades, including the knife used to stab Keith Lyon, 14.

On the weapon are two types of blood, the victim's and possibly the killer's.

Detectives are hoping modern DNA and forensic tests will now identify whose blood it is.

All those suspected at the time may be obliged to provide samples for DNA matching.

Also discovered was a cigarette butt found at the scene of the killing which may also carry a DNA profile.

Sussex Police, who described the find as "the best news we've had so far", were highly embarrassed when the knife and other items went missing.

They were discovered during renovation work.

Detective Inspector Bill Warner, of Brighton and Hove CID, said: "There is no doubt this is a significant breakthrough and we will be sending items for analysis.

"This is exactly what we have been waiting for. I am delighted the items have been found although I never believed they had been lost for good.

"People involved in a murder inquiry that long ago have retired and some have died.

"It was not surprising some of the material was put somewhere safe. We just didn't know where that safe place was."

Keith, the son of bandleader, the late Ken Lyon, was murdered on downland in Woodingdean, Brighton, in 1967.

Keith's mother, Valda Lyon, 84, from Hove, was angry when police first told her the knife had been lost.

Last night, she said she was delighted it had been found, but added: "I am not keeping my hopes up too high. I have been here before and I have been disappointed before.

"I know my late husband Ken was terribly sad knowing he would die before police could find out who killed our son.

"I am coming up 85 now and I am hoping it will be established before I go."

Keith's brother Peter, 42, said: "This is brilliant news - this is what we have been hoping for for a long time."

He and his mother have vowed not to rest until Keith's killer has been brought to justice.

Mrs Lyon said: "Keith's murder has ruined our lives and there is not a day that goes by when we don't think about him, what happened to him and what he might be doing now if he was alive."

They are hoping a £10,000 Sussex Police reward will encourage someone to come forward with clues to crack the case.

Anyone with information should contact Mr Warner on 0845 6070 999.