The West Belfast-based republican was a key figure in the organisation during the peace process.

Sinn Féin President Gerry Adams today expressed his "deep sense of personal loss".

Keenan was a former member of the IRA's Army Council who received an 18-year prison sentence in 1980 for conspiring to cause explosions.

The 66-year-old father-of-six was involved in talks on weapons decommissioning with Canadian General John de Chastelain.

An apprentice electrical engineer, he joined the IRA in 1968, following violence in Belfast and Londonderry and at one stage was described as the single biggest threat to the British Army.

Tony Blair's chief of staff, Jonathan Powell, said Mr Keenan was once " the single biggest threat to the British state", but was " instrumental in bringing the IRA round to the political strategy" and was also the man who eventually achieved decommissioning.

In the early 1970s he controlled the arms of the Belfast IRA as quartermaster and was later accused of organising the bombing campaign in England.

He spent 25 years on the run and 16 years in jails across England. He resigned from the Army Council in 2005 due to ill-health.

Former Chief Constable Sir Ronnie Flanagan once said that if you had Keenan's pulse then you had the pulse of the IRA.

A senior security source, speaking today, said Keenan played a huge role in convincing IRA rank and file of the move towards peace. He sat in the public gallery at Stormont and watched as Martin McGuinness led Sinn Fein into government with Ian Paisley in May last year.

"He was an enormous part of the problem and he was part of the solution. In fact he was key to the solution.

"If he didn't give the nod, it wasn't going there," the source said.

Sinn Féin President Gerry Adams described Keenan as having been crucial in securing IRA support for the peace process. He said the death would come as a shock to all republicans.

"Brian was a formidable republican leader over 40 years of activism. He was a man of tremendous energy, even in the face of a debilitating illness.

"Brian’s strong endorsement of the Sinn Féin peace strategy was crucial in securing the support of the IRA leadership for the series of historic initiatives which sustained the peace process through its most difficult times.

"He made an incalculable contribution to the republican struggle,” he said.

“Brian will be greatly missed by his family and friends and by the many republicans who over the years have been touched by his generosity, friendship, and humour,” he added.

Mr Adams said he was a good friend and steadfast republican and he extended his condolences to Keenan's wife Chrissie, his sons and daughters, Bernadette, Annemarie, Chrissie, Frankie, Sean and Janette and his grandchildren.