The British Government is using an investigation into the murder of Belfast solicitor Pat Finucane to block an inquiry into the killing, Sinn Fein has claimed.
Sir John Stevens met Mr Adams for the first time in London
Sinn Fein President Gerry Adams made the claim during a meeting in London on Tuesday with Metropolitan Police Commissioner Sir John Stevens.
It was Mr Adams' first meeting with the police commissioner.
A Sinn Fein spokesman said they discussed the Stevens investigation into the killing of Pat Finucane and the wider issue of collusion.
The hour-long meeting took place in a London hotel on Tuesday.
Mr Finucane was shot dead by loyalist paramilitaries, the UDA, at his home in Belfast in 1989.
Sir John investigated the killing and concluded that there had been collusion between security forces and loyalist paramilitaries.
In April, the government confirmed that the retired Canadian judge Peter Cory recommended a public inquiry into Mr Finucane's murder.
Finucane family want the government to hold a public inquiry
However, the Secretary of State Paul Murphy said that couldn't happen now because of a case before the courts and the continuing Stevens investigation.
A Shankill loyalist Ken Barrett is charged with Mr Finucane's murder.
After the meeting, Mr Adams said he had told the police commissioner that the British Government was using his inquiry, and the trial of the Belfast loyalist Ken Barrett, as excuses to obstruct the establishment of a public inquiry into the Finucane killing.
"The institutional use of collusion for over 30 years has led to the deaths of hundreds of people, the maiming of thousands more and the terrorising of an entire community," he said.
"The Pat Finucane case is at the heart of all of this.
"That is why the British system is so determined to block a public inquiry."
SDLP Justice Spokesperson Alban Maginness said the meeting was an important step forward in the debate on policing.
"I think it is significant that Sinn Fein has realised that in seeking the truth on the murder of Pat Finucane, engagement is likely to achieve more than boycotting," he said.
The Finucane family has taken a legal challenge aimed at forcing the British Government to set up a public inquiry into the murder.
Mr Adams has met members of the Stevens investigating team before but this was his first meeting with the metropolitan police commissioner.