LONDON (Reuters) - A child killer, who became Britain's longest-serving prisoner, has died in custody after 55 years behind bars, the government said on Tuesday.

John Straffen, 77, gained notoriety in 1952 after escaping Broadmoor high-security hospital and within hours murdering a five-year-old girl, Linda Bowyer.

At his trial in 1952 he was sentenced to death, but the sentence was reduced to life imprisonment by the then Home Secretary, David Maxwell Fyfe, on the grounds Straffen was a "feeble-minded person".

He was in Broadmoor because he had been deemed unfit to stand trial in 1951 for the murder of Cicely Batstone, nine, and six-year-old Brenda Goddard.

Solicitors acting for Straffen called for his case to be re-opened in 2001, saying he should not have stood trial, as he had a mental age of nine and a half.

The Ministry of Justice said Straffen, from Bath, died in Frankland Prison, near Durham, on Monday.

After Straffen's escape from Broadmoor in 1952, a siren system was installed to alert local residents if an inmate had escaped.

(Reporting by Avril Ormsby; Editing by Peter Griffiths)