Published Wednesday, 07 April 2010
The retired RUC Detective, Alan Simpson, made the claim in a book entitled Duplicity and Deception, which is due to be published on Thursday.
Mr Finucane, a high profile catholic solicitor, was gunned down as he sat eating a meal with his family at their north Belfast home in 1989.
Three major inquiries headed by senior policeman John Stevens concluded that there was collusion between the loyalist killers and the RUC's Special Branch.
In 2004, Canadian judge Peter Cory recommended a public inquiry but it has yet to begin.
Detective Superintendent Alan Simpson, who retired from the RUC in the 1980s, is extremely critical of elements within Special Branch.
"My golden rule is the police must never lose the moral precedence when dealing with terrorists or criminals", he told UTV.
When asked if he felt that Special branch had lost that "moral precedence", he responded:
"Towards the end with a very small number of them. Unfortunately when the history of the RUC is looked at in 100 years time it will still be a stain on the 300 and more colleagues who were murdered, which is a case of deep regret for me."
The Detective Superintendent has said he vividly remembers the murder scene and knew right from the start that the killing was politically sensitive.
"The murder happened on a Sunday evening and on the Tuesday I got a visit from my Assistant Chief Constable in charge of the CID to the incident room.
"As I showed him all that was going on and as I showed him out to the car he turned to me and said: 'Alan if I were you I wouldn't get too deeply involved in this one.'
"I thought he was basically saying we've a lot on that the minute. 'Pat Finucane is from a strongly Republican family don't waste too much time or resources in this'. That didn't hit me either," said the Detective Superintendent.
"It wasn't until the years rolled on and the Stevens Inquiry concluded there had been collusion in the Finucane murder and he concluded also that if I had been given all the information available at the time I could have cleared the murder at an early stage."
The Finucane family released a statement in response to the lastest claims.
"This new information from Alan Simpson is very important and supports our campaign for an independent public judicial inquiry into Pat's murder," Geraldine Finucane said on Wednesday evening.
The statement went on: "Mr Simpson's new information emphasises that the full truth about Pat's murder can only be obtained by identifying relevant witnesses and requiring them to produce their relevant documentation for inspection and to give evidence in public."