The Sinn Fein president has denied any involvement in the murder

He was found shot dead in his cottage in Ireland in April 2006

Sinn Fein leader Gerry Adams has denied sanctioning the murder of a former leading party official who had been revealed as a spy for MI5.

Denis Donaldson was found shot dead inside his cottage near Glenties in Co. Donegal, Ireland, in April 2006.

It was believed that dissident group the Real IRA carried out the killing, but claims have recently been made that the death was ordered by republican and alleged Provisional IRA member Thomas 'Slab' Murphy.

Sinn Fein president Gerry Adams, right, has denied sanctioning the murder of former MI5 informant Denis Donaldson, left, who was killed in April 2006

Mr Donaldson was found shot dead inside his cottage, pictured, months after it was revealed he was working for the British Government inside the IRA and Sinn Fein

Mr Adams (right) pictured with Deputy First Minister of Northern Ireland Martin McGuinness (centre) made the announcement about Mr Donaldson's (left) MI5 involvement at a press conference in December 2005, pictured

The shock allegation was made by a man claiming to be a former Special Branch agent within the IRA to BBC Northern Ireland's Spotlight programme.

He also told the BBC Mr Donaldson – revealed as an MI5 informer by Mr Adams in December 2005 – had unearthed a 'spy ring' within the Northern Irish Assembly in 2002, that the IRA held maps and security codes to the British Government's Northern Irish Office and that the IRA also possessed personal information about all serving prison officers and several unionist politicians.

The anonymous informer, known as 'Martin' said: 'I know from my experience in the IRA that murders have to be approved by the leadership.

'Gerry Adams, he gives the final say.'

He added the Real IRA only claimed responsibility for the death in 2008 because 'someone might take them more seriously'.

The claim was rejected by the Sinn Fein leader, who has always denied he was in the IRA. His lawyer told the programme his client had no knowledge of or involvement in Mr Donaldson's death and denied being consulted about it.

Mr Adams's party colleague Gerry Kelly dismissed the documentary as a 'collection of discredited conspiracy theories'.

'The programme makers have had no regard for the feelings of families of the victims of the conflict, including the family of Denis Donaldson,' said the North Belfast Assembly member.

Mr Adams has consistently denied he was a member of the IRA.

The programme also aired claims that Murphy 'ordered the killing to maintain army discipline' following the revelation that Mr Donaldson was working for MI5.

It was also alleged that Irish republican Thomas 'Slab' Murphy, who was jailed for tax evasion earlier this year, ordered the killing

Murphy, 67, was jailed earlier this year for tax evasion and sentenced to 18 months in prison.

The BBC programme said it tried to contact Murphy's representatives to put the claims to him but received no reply.

The Real IRA split from the Provisional IRA in 1997 after rejecting the ceasefire called against Great Britain in July of that year. It has been designated a terrorist organisation by the British Government.

The police's handling of the murder investigation into Mr Donaldson's death is being investigated by the Police Ombudsman, with a report set to be published later this year.

During the Troubles it is believed there were at least 800 informers for the British Government within the IRA and Sinn Fein.

Raymond White, a retired Royal Ulster Constabulary assistant chief constable and Special Branch Officer, told the BBC well-placed agents could subtly influence decisions made by IRA leadership and 'slowly damage' the group from inside.

He said: 'Agents had to be selected and, if possible, placed or manipulated into certain positions and allowed to develop and grow.'

'If you looked upon agents and those, they're sort of a cancer within, a slow-growing cancer - they can sort of infect other parts of the system.'