Giles Van Colle was shot three times by Daniel Brougham
Law Lords are expected to rule later on whether a police force breached the human rights of a witness who was murdered before he could give evidence.
Hertfordshire Police failed to protect Giles Van Colle, 25, shot dead in Mill Hill Broadway, north London, in 2000.
Police counsel said his killer's actions could not have been foreseen and did not involve human rights laws.
The Law Lords' ruling could affect policing and government at national and local level, it has been claimed.
The House of Lords is also due to rule on a second related case involving the level of protection afforded by police to a Sussex man who was attacked by his former partner.
Stephen Smith had repeatedly warned Brighton police that he was in danger from Gareth Jeffrey, who later attacked him with a claw hammer.
But the case which could have far reaching legal consequences involves Mr Van Colle and a finding by the the High Court in March 2006 that Hertfordshire Police violated Articles 2 and 8 of the Human Rights Act.
Article 2 is often referred to as the right to life while Article 8 is described as the right to respect for private and family life.
Mr Van Colle had been due to give evidence against Brougham, a former employee, who was sacked for stealing.
But Monika Carss-Frisk QC, counsel for Mr Van Colle's family who are also appealing against the halving of their damages to £25,000, said police "did not even address the need to protect this crucial witness".
Three appeal judges rejected a challenge by the force that they had violated his human rights by failing to "discharge the positive obligation of the police" to protect his life.
The Home Office, Equality and Human Rights Commission and four charities, Inquest, Justice, Liberty and Mind, are involved in the actions before the court.
Previously the Equality and Human Rights Commission, has said a judgment against Hertfordshire Police would result in "significant policy and procedural change within the police force".