David Cameron told the family of Belfast solicitor Pat Finucane others would not let a public inquiry into his murder happen, according to High Court papers.
The lawyer's widow Geraldine also claims a mystery intervention led to an apparent change of mind by the Prime Minister over the holding of a full independent inquiry.
Details emerged as lawyers for the family yesterday cleared the first stage in their legal challenge over the refusal to order a public inquiry.
A judge granted leave to seek a judicial review after being told the Government was not opposing the preliminary application.
The case has been listed for a full hearing over three days in May.
During a brief hearing at the Northern Ireland High Court, the Finucanes' legal team also dropped their interim bid to stop Mr Cameron's alternative plan for a senior lawyer to examine the case.
Sir Desmond De Silva is now expected to continue his review of the killing and report back by the end of the year.
Mr Finucane was gunned down at his north Belfast home by the loyalist Ulster Freedom Fighters in 1989.
His family have campaigned for a full independent inquiry and believed they were set to achieve that when they went to Downing Street to meet the Prime Minister last October.
Instead, they were shocked to be told that a review rather than an inquiry would take place.
In an affidavit filed as part of their legal challenge, Geraldine Finucane states: "I felt, and still feel, that the Prime Minister's actions in asking us to travel to London at our own expense to hear a decision that he must have known would not be welcomed were bewildering.
"While I still feel that we were treated cruelly by the Prime Minister and Secretary of State, on reflection I consider that the only explanation for the apparent change of mind could be an intervention or interventions by a person or persons unknown that took place between the date on which the meeting was arranged and the holding of the meeting." Mrs Finucane adds: "This view was supported by a comment made by the Prime Minister during the meeting when he said 'It is true that the previous administration could not deliver a public inquiry and neither can we. There are people in buildings all around here who won't let it happen'."
Northern Ireland Secretary of State Owen Paterson has said he believes that the De Silva review is the best mechanism available to get to the truth of what happened in the case.
Mrs Finucane attended court along with her son John and family solicitor Peter Madden.
Following the judge's decision she said: "I am very pleased that he has granted us leave to go forward to a full judicial review hearing and I think the significant aspect of it was that it was completely unopposed (by the Government).
It was surprising, but a very pleasant surprise for a change."