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

Mr 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.

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.

Mr Keenan once masterminded the organisation's bombing campaign in Britain.

With his international terrorist contacts, it was Keenan, son of a former RAF officer but arguably the top Provo, who Downing Street feared the most.

Jonathan Powell, Tony Blair's former chief of staff, admitted Keenan was "the single biggest threat to the British state".

The feared Belfast republican, 66, oversaw the bombing campaign in England and had links to Libya, Lebanon and Syria.

Keenan's fingerprints were discovered in 1975 at the hide-out of the Balcombe Street Siege gang in London and he was arrested four years later and sentenced to 18 years in prison.

The group attempted to bomb the home of former prime minister Sir Edward Heath and attacked railway stations, hotels and restaurants.

He resigned from the Army Council in 2005 due to ill health.

Sinn Fein president Gerry Adams said: "Brian was a formidable republican leader over 40 years of activism."

He added: "Brian Keenan's strong endorsement of the Sinn Fein 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."