His father, David, believes there was a conspiracy behind the murder. He handed in a report detailing the circumstances surrounding the killing at Downing Street on Wednesday.

Jane Winter: A very high responsibility on the state

The report was prepared by British-Irish Rights Watch, a London-based human rights group which has supported calls for inquiries into the deaths of Portadown man Robert Hamill, who died after being attacked by loyalists, and Rosemary Nelson, a lawyer killed by a loyalist car bomb.

BIRW director Jane Winter said the primary responsibility for Billy Wright's murder lay with the three INLA men involved in the shooting.

"However, that such a notorious figure could have been so easily be murdered within the confines of a prison, places a very high responsibility on the state."

Mr Wright told the BBC that important details about the events on the morning of his son's murder had been witheld from him.

"I find it remarkable that, even today, I do not know the name of the governor that was in charge of my son's welfare that particular morning.

"The government won't even tell me who was in charge of security that morning."

A copy of the report has already been sent to the Irish government.

Killers released

Repeated requests by Mr Wright for a meeting with the prime minister or secretary of state have been turned down.

Mr Wright's visit to Downing Street comes less than a week after his son's killers secured early release under the terms of the Good Friday Agreement.

Christopher McWilliams told reporters he would support an inquiry into the killing but said unionist politicians making the demand were "hypocrites".

Billy Wright: Murdered inside the Maze prison

Billy Wright was among the most feared loyalist paramilitary figures. He was involved in the Ulster Volunteer Force before leaving the organisation to form his own splinter group, the Loyalist Volunteer Force.

An inquest into Wright's killing in 1999, concluded it had been carried out by three INLA men in an elaborate premeditated and pre-planned attack.

He was shot as he sat in the back of a prison van.

His killers managed to smuggle two handguns into the prison and were able to climb out of their prison block and over a roof before jumping into the yard where the van was parked.