A couple who have fought for five years for justice for their murdered son will have to face the full might of the government at a House of Lords appeal next month.

Irwin and Corinne Van Colle have already won two judgments against Hertfordshire Police in the High Court and the Court of Appeal. On both occasions, judges decided that the police force had failed to protect their son Giles, who was murdered by one of his former employees in November, 2000.

Next month, the couple, who live in North London, face what could be the final round in their long fight, begun in 2003, to prove that their son died unnecessarily, when Hertfordshire Police will ask the law lords to overturn the judgments made against them.

But this time, Hertfordshire Police will not be alone in challenging the decisions, since the law lords have given permission for the Home Office to make an intervention in the hearing. This means it will be able to make representations about what it believes will be the implications for every police force in the country if the Appeal Court decision is upheld.

While he would not comment ahead of the House of Lords hearing, Mr Van Colle said after their Court of Appeal judgment last year: “This will have a significant impact on the public sector. The welfare of witnesses will be better served than before, which will help to protect many from intimidation. Our case has clarified the law, and though we will suffer his loss for the rest of our lives, Giles’s death was not in vain.”

Mr Van Colle was just 25 when he was murdered in November, 2000, by former employee Daniel Brougham. The young optician was due to give evidence in a theft case against Brougham and had told the officer in charge of the case, Det Con David Ridley, that Brougham had threatened his life. But the policeman did nothing to protect Mr Van Colle.

Mr and Mrs Van Colle sued Hertfordshire Constabulary under human-rights legislation and won a judgment by Mrs Justice Cox that Hertfordshire police should have acted to protect their son. They were also awarded £50,000 damages, although the couple made it clear that their decision to pursue Hertfordshire was not about money.

Hertfordshire appealed and, almost exactly a year ago, three Appeal Court judges, led by Master of the Rolls Sir Anthony Clark, said they agreed unanimously with Mrs Justice Cox’s decision. They concurred that Hertfordshire had breached Mr Van Colle’s human rights by failing to protect him from his killer. However, they halved the monetary award.

Now Hertfordshire has one last throw of the dice at the highest court in the land. If the law lords decide for the Van Colles, Hertfordshire can go no further. If Hertfordshire wins, though, Mr and Mrs Van Colle can take the case to the European Court of Human Rights in Strasbourg.