The BBC's Northern Ireland Correspondent Denis Murray has learned the claim will be made in a report by the Metropolitan Police Commissioner Sir John Stevens, which he is due to make public on Thursday.

The report into alleged collusion between the security forces and loyalist paramilitaries has also found that military intelligence in Northern Ireland actually prolonged the Troubles.

It suggests one branch of military intelligence was out of control and its activities were disastrous.

It is understood the latest report, called Stevens Three, will find that members of the RUC and army colluded with the largest loyalist paramilitary group, the Ulster Defence Association (UDA), to murder Catholics.

The report, which centres on the murder of Pat Finucane in 1989, will be delivered to the chief constable, Hugh Orde.

The Finucane family has always believed the security forces were involved in his murder and have dismissed the report.

His widow, Geraldine, said a full judicial inquiry was the only way to deal with the issue.

Mr Finucane, a high-profile Catholic solicitor, was shot dead by the UDA in front of his family at his north Belfast home.

His son, Michael, alleged the system of collusion came about because the government wanted certain people "out of the way".

"I know this because I have spent 14 years examining the evidence in the case and I am a victim of that policy - my father was one of the people murdered.

"The success of the policy, in government terms, is measured by the numbers of people they were able to kill and get away with it.

"The only thing that I don't know, and it certainly hasn't been determined anywhere, is the exact total number of people murdered as a result of this policy of collusion."

The report also says its inquiries were obstructed by police and army officers, and vital evidence was concealed and destroyed.

Since 1989, Sir John Stevens has been investigating allegations that elements within military intelligence and the RUC's Special Branch were colluding with loyalist assassination squads.

During the course of the latest Stevens inquiry, the activities of the Army intelligence Force Research Unit were investigated.

It recruited Brian Nelson as its agent at the top of the UDA.

His role was to gather information on murder targets.

Nelson, who died last week, insisted his handlers knew in advance that Pat Finucane was being targeted.

The Stevens report says his murder could have been prevented.

The head of the FRU at the time was Gordon Kerr, who is now an army brigadier serving in Iraq.

Last February, prosecution papers were prepared relating to Brigadier Kerr and are one of 20 files that have been sent to the Director of Public Prosecutions.

The Stevens investigating teams found obstruction and even harassment from both the army and elements of the RUC's special branch.

They say a fire at their offices in 1990 was arson and they feel that throughout their inquiries, they were spied on and betrayed by police and army colleagues.

Sir John Stevens is still determined to try to bring Pat Finucane's killers to justice - and he is still investigating just how far up the chain of command the collusion might have gone.

But BBC Northern Ireland correspondent Denis Murray says: "Neither he nor his officers ever expected to uncover a can of worms on the scale they have."