The parents of two French students brutally killed in London are demanding compensation from the British government for mistakes that could have prevented the murders, their lawyer said Saturday.

Laurent Bonomo's parents and the mother of Gabriel Ferez wrote to the justice ministry and London's Scotland Yard police on Friday, setting out their claim for negligence and human rights violations, lawyer Anna Mazzola told AFP.

The letter is the first step in a civil case against the government, which now has until January to respond, she said.

"If at that stage they back down and concede that they are legally liable for Laurent and Gabriel's deaths then we can enter into negotiations with them" to reach a settlement, she said.

"But if they still deny that they are responsible for the deaths, then we will go to the next stage of going to court."

Ferez and Bonomo, both 23 and in London on a three-month research placement, were tied up and stabbed a total of 244 times in Bonomo's south London apartment in June 2008. The flat was then set on fire to try to destroy the evidence.

Two men, Daniel Sonnex and Nigel Farmer, were handed life sentences for the attack, described in court as an "orgy of bloodletting".

It was later revealed that Sonnex, who had a long and violent criminal history, should have been in jail for breach of his probation at the time but was free due to a series of mistakes in the criminal justice system.

London's top probation officer quit over the affair, and Justice Secretary Jack Straw personally apologised to the victims' families.

Mazzola insisted the men's parents were not interested in the money, saying "this is really more about getting accountability".

"They are claiming compensation but they are also seeking a declaration and confirmation that the systems will be changed," she said.

Bonomo's father, Guy, told AFP: "We want to make sure people don't forget too quickly, and keep making noise about this case, which embarrassed the British authorities."