Material gathered as part of an investigation into security force collusion on both sides of the Border was seized and destroyed by MI5, according to a BBC documentary to be broadcast this evening.

The latest episode of Spotlight on the Troubles: A Secret History reveals that hard drives were taken from the London offices of the retired Canadian Supreme Court judge Peter Cory in 2002 by members of MI5 “in the interests of National Security”.

These were “wiped clean and returned” by MI5, said presenter Mandy McAuley. MI5 explained it had been concerned the inquiry’s computer systems were insecure and a leak could endanger informers. Judge Cory had kept backup copies of the material, and did not believe any information had ultimately been lost as a result of the raid.

Judge Cory had been appointed earlier that year to lead an investigation into allegations of security force collusion into a number of high-profile murders, including the loyalist killing of solicitor Pat Finucane in 1989.

His widow, Geraldine Finucane, told the programme that she believed that collusion between the British government and loyalist paramilitaries in the killing of her husband went to the highest levels.

“I think it went right to Cabinet level,” she said. “I think the prime minister of the day knew exactly what was going on.”

She said Judge Cory had told her that “papers marked for Cabinet eyes only involved the collusion and the killing of my husband”.

“There’s still something there that needs to be exposed,” said Mrs Finucane.

The programme also revealed that victims campaigner Willie Frazer was former UDA commander Johnny Adair’s contact within the loyalist paramilitary group Ulster Resistance in the 1990s, and was its key distributor of automatic rifles and rocket launchers.

Adair told the programme the weapons it received from Ulster Resistance were “a Godsend” and led to an escalation of loyalist violence.