Giles Van Colle was shot dead at the back of his shop

Giles Van Colle, 25, was shot dead outside his shop in Mill Hill Broadway, north-west London, in November 2000.

Killer Daniel Brougham had threatened the optician because he was due to testify against him in a theft trial.

If successful, the case could set a precedent for more people to sue the police for not protecting them.

Mr Van Colle was murdered as he sat in his car at the back of the shop, just days before the trial of Brougham, 36, was due to take place.

Brougham was convicted of murder in 2002

Papers relating to the case show that prior to the shooting the killer rang his victim and said: "I know where you live. I know where your businesses are and where your parents live. If you don't drop the charges you will be in danger."

Mr Van Colle's parents Irwin and Corinne are bringing the case under the right to life human rights laws.

Their counsel, Monica Carss-Frisk QC, told Mr Justice Wakerley at the high court that despite threats and the intimidation of Mr Van Colle, Hertfordshire Police did not protect him.

She went on to say the police accepted that Det Con David Ridley, who was in charge of the case, did fail in his duties in the sense that was established by a misconduct panel.

Ms Carss-Frisk told the court that although the police also accepted Brougham should have been arrested or contacted, the force did not accept they had breached the victim's or his family's rights.

Brougham was found guilty of murder at the Old Bailey in 2002.

Hertfordshire Police said they would not comment until after the case.

The High Court hearing continues.