The family of Jean Charles de Menezes are challenging a decision not to bring charges over his death at the European Court of Human Rights (ECHR).
Relatives of the Brazilian will take their case to the Strasbourg court on Wednesday, 10 years after he was mistaken for a suicide bomber and shot dead by police marksmen on a London Tube train.
His family claim the test to prosecute applied by the Crown Prosecution Service (CPS) - that there should be sufficient evidence for a "realistic prospect" of conviction - is too high a threshold.
Family members of Jean Charles de Menezes including Erionaldo da Silva, left, Alessandro Peirera, center, and Vivian Figueiredo stand as the court enters at the European Court of Human Rights in Strasbourg on Wednesday
Patricia da Silva, a cousin of de Menezes who lodged a case against the CPS, said: "For 10 years our family has been campaigning for justice for Jean because we believe that police officers should have been held to account for his killing.
"Jean's death is a pain that never goes away for us.
"Nothing can bring him back but we hope that this legal challenge will change the law so that no other family has to face what we did."
De Menezes was shot dead by Metropolitan Police firearms officers at Stockwell Underground station in south London on July 22 2005.
Da Silva pictured at a press conference
The following year the CPS announced that no individual should be charged.
In 2007 the Met was fined £175,000 after being convicted of breaching health and safety laws.
An inquest jury later rejected the police account of the shooting and returned an open verdict. The coroner had already ruled out a verdict of unlawful killing.
Harriet Wistrich, a solicitor representing the family, said: "The officers who shot Jean Charles have a defence if they had an honest belief that they were under imminent threat, even if they were mistaken and their mistake was wholly unreasonable."
The CPS has declined to comment ahead of the hearing.
Jean Charles de Menezes was shot dead by police at Stockwell Tube station