Britain's appeal court on Thursday quashed the conviction of a "simple-minded teenager" who was hanged 45 years ago for the murder of a policeman.

Three of Britain's top judges said Derek Bentley, who was 19 when he died but had a mental age of only 11, had not had a fair trial and should never have been hanged.Bentley played no direct part in the 1952 killing, but was found guilty largely because police alleged he had shouted "let him have it, Chris," to a younger friend who shot the policeman dead.

Police had caught Bentley and Christopher Craig trying to break into a warehouse and were chasing them across a rooftop when the killing took place.

Craig was only 16 at the time, too young to receive the death sentence, and so he was jailed instead. He served 10 years.

The appeal court judges concluded that Bentley had never said the incriminating words "let him have it, Chris" and didn't even know his robbery accomplice had a gun.

They also said the jury had never been told that Bentley was mentally retarded.

The conviction was "such as to deny the appellant that fair trial which is the birthright of every British citizen," Judge Thomas Bingham said in the court ruling.

The ruling was a poignant victory for Bentley's family, who had fought for nearly half a century to clear his name.

Bentley's sister, Iris, who devoted much of her life to the campaign, suffered endless setbacks as successive interior ministers turned down requests for an appeal.

She died of cancer last year, without ever knowing that her brother's case would finally reach the appeal court.

Iris Bentley's daughter, Maria Bentley Dingwall, who held up the judges' ruling in triumph on the steps of London's High Court on Thursday, said the family's celebrations would be muted.

Derek Bentley's gravestone still bears the inscription "a victim of British justice."