Image caption Denis Donaldson (cream jacket) was a former senior Sinn Féin official who became an informer

A man who says he was a former IRA and Sinn Féin member turned British agent has claimed Sinn Féin leader Gerry Adams sanctioned the killing of Denis Donaldson.

He made the allegation to BBC NI's Spotlight programme on Tuesday.

Mr Donaldson was shot dead months after admitting in 2005 that he had been an MI5 agent for more than 20 years.

Three years later, the Real IRA said it was responsible. Mr Adams has denied any involvement in the killing.

Media playback is unsupported on your device Media caption Gerry Adams has strenuously denied the Spotlight claims

"I specifically and categorically refute these unsubstantiated allegations," said Mr Adams.

The Spotlight allegations were "part of the British security agencies ongoing attempts to smear republicans and cover-up their own actions", he added.

"There is a need for all these agencies to fully co-operate with the Ombudsman's investigation into the role of the police in the events that led to the killing of Denis Donaldson and for the Gardaí to expedite their investigation to bring those responsible to justice."

The Sinn Féin leader said he would "continue to support the family of Mr Donaldson to achieve truth and justice".

Ulster Unionist MLA Doug Beattie said Mr Adams should take legal action against the BBC if the allegations were untrue.

"If not, he needs to explain why not," said Mr Beattie. "All we have heard so far is a weak denial."

Murder sanctioned?

Mr Donaldson had worked for Sinn Féin as an administrator at Stormont. He was killed at a remote Donegal cottage in 2006.

Image caption Mr Donaldson was shot dead at a cottage in the Glenties, Donegal, in 2006

A man who says he worked as an informer for the intelligence branch of the police told Spotlight that Mr Adams sanctioned the murder.

The former spy was in the IRA and Sinn Féin. He cannot be identified because of fears about his safety.

What Spotlight was told

Agent: I know from my experience in the IRA that murders have to be approved by the leadership. They have to be given approval by the leadership of the IRA and the military leadership of the IRA.

Presenter: Who are you specifically referring to?

Agent: Gerry Adams, he gives the final say.

The informer told Spotlight that murders had to be approved by the political and military leadership of the IRA.

Image caption BBC Spotlight spoke to an informer who said he believed Gerry Adams sanctioned Denis Donaldson's murder

In a statement, Mr Adams' solicitor said the Sinn Féin leader "has no knowledge of and had no involvement whatsoever in the killing of Denis Donaldson".

He added that Mr Adams "categorically denies the unsubstantiated allegation that he was consulted about an alleged IRA army council decision or that he had the final say on what had been sanctioned".

Days after the murder, the IRA said it was not involved in Mr Donaldson's death.

But security sources have told Spotlight that intelligence received following the killing contradicted the IRA's denial.

You can watch BBC Spotlight: Spy in the IRA now on BBC iPlayer.