The informant told police he had overheard an acquaintance complaining that a Wan Chai hostess had failed to show up at his flat after he had paid for sexual services.

When he read Castro's Western accomplice had been driven to murder by similar circumstances, he decided to report the conversation to police.

Castro's first trial collapsed on June 25 to allow officers to investigate the tip-off. The jury had been listening to evidence for more than a week.

But police who interviewed the suspect, his employer and his family members concluded he had played no part in the killing.

'The file remains very much open,' a spokesman said.

Anyone with any information that could help the investigation is encouraged to contact District Crime Squad 1 at Central Police Station.

Wilma Gutgutao had two Western boyfriends, neither of whom is a suspect. One of them appeared as a witness in the first trial, and took the unusual step of asking to testify anonymously.

Mr Justice Michael Stuart-Moore allowed the stockbroker to withhold his first name. He was identified simply as 'Mr Russell'.