Detectives may have to abandon the investigation into Britain's longest missing person's case after being told they have insufficient evidence to charge a key suspect.

It is the 30th anniversary of the disappearance of 13-year-old Genette Tate who went missing from a lane in Devon on August 19, 1978.

In 2005, Devon and Cornwall Police interviewed convicted child murderer Robert Black, who in 1994 was given ten life sentences for the abduction and murder of three young girls and the abduction of a 15-year-old girl.

After the interview the force sent a file to the Crown Prosecution Service who announced on Monday that there was insufficient evidence to bring criminal proceedings.

Genette's father, John Tate, was understood to be "in despair" at the news. He said: "It's something that you can't imagine. You have so many sleepless nights that it must have an effect.

"You mind doesn't think properly. You've always got it there ticking away. There is only one thing I want to know from Black is where is she? I don't care much who did it any longer. I just want to know, where is she so we can bury her properly."

Black was also interviewed by the Devon and Cornwall force in 1996 and 1998 and, although officially the case will remain open, the police are understood to have few remaining leads to investigate.

A spokesman for the force said: "The investigation remains open.

"The force is aware that the 30th anniversary is approaching and would ask anyone with further information concerning this unsolved crime to contact us."

Over the years the Genette file has grown so large it is kept in a 12ft by 10ft document cage at the Devon and Cornwall force headquarters.

The mountain of paperwork includes more than 20,000 cards in a filing system.

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