Law Lords are reviewing a court decision to hold police legally liable for failing to protect a witness before he was murdered.

Giles Van Colle, 25, was shot dead at close range behind his shop, GVC Optometrists in Mill Hill Broadway, by former employee Daniel Brougham in November 2000, days before he was due to testify against him in a theft trial.

Brougham was convicted of Giles's murder in March 2002 and jailed for life.

His parents, Irwin and Corrine Van Colle, who live in Wembley, sued the police under the Human Rights Act. They argued that Hertfordshire Police, particularly Detective Constable David Ridley, who was leading the case against Brougham, should have taken steps to protect their son.

Giles had reported threatening phone calls from Brougham and his car had been set alight.

In a telephone call in October 2000, Brougham told Giles: "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."

At a police disciplinary tribunal in 2003, Det Con Ridley was found guilty of three charges of failing to perform his duties diligently, failing to investigate thoroughly the intimidation of witnesses, and failing to arrest Brougham. He was fined five days' pay but was later promoted to temporary detective sergeant, having successfully passed his sergeants' exam.

In March 2006, the Van Colles were awarded £50,000 in a landmark High Court case against Hertfordshire Police.

The High Court ruled that had Mr Van Colle been given police protection after being threatened by Brougham, he may not have been killed.

This award was reduced in 2007 to £25,000 by the Court of Appeal, but the Master of the Rolls, Sir Anthony Clarke, dismissed an appeal by Hertfordshire Police, which did not want to admit liability.

Now Hertfordshire Police is contesting the appeal court judgement to hold them liable for Giles's death.

A spokesman said: "The implications of the ruling have potentially widespread implications for policing in Britain and that's the reason why it's gone to the highest level."

Law Lords at the House of Lords will consider the case until May 22, with a decision due in eight weeks.