One of Britain’s most prominent criminal lawyers has become the legal adviser for detectives investigating an army agent operating in the top ranks of the IRA, the Observer can reveal.

The spy, known as “Stakeknife” and once described as the “jewel in the crown” of British military intelligence, allegedly was implicated in acts of murder and torture while he ran the IRA’s internal security squad during the Troubles.

The appointment of Jonathan Laidlaw QC, who defended News International boss Rebekah Brooks and has prosecuted Al-Qaida bombers, is being seen by legal experts as a sign of “serious intent” to put the army’s long-serving informer in the dock.

Reports have widely named Freddie Scappaticci, 73, as Stakeknife, the man in charge of the IRA’s spy-catching unit that interrogated and killed members of the Provisionals accused of being informers. Since 2003, Scappaticci has denied he was “Stakeknife”.

But it is alleged that, while the unit abducted, sometimes tortured and shot informers dead, Stakeknife was a double agent passing on high-grade information to his handlers in British military intelligence.

The Observer has learnt that Operation Kenova, the multimillion pound inquiry into the activities of Stakeknife, recruited Laidlaw to help its detectives draw up the legal papers needed to bring any prosecution.

Legal sources have told the Observer that Laidlaw’s experience in preparing charges for long-running investigations and ensuring cases are brought to trial will be crucial to Operation Kenova bringing the alleged army agent to court back in Northern Ireland.

They said that Operation Kenova needed someone of Laidlaw’s calibre to “enable them to battle against the resistance both the Ministry of Defence and MI5 will put up to stop Stakeknife appearing in court. It shows serious intent to do so.”

Both the security forces and the leadership of the republican movement may have concerns if Stakeknife is forced to give evidence about his alleged role as an agent inside the IRA’s most secretive unit.

There are concerns that testimony could reveal the identities of alleged handlers and any alleged willingness to “sacrifice” low-level informers to protect their agent placed in the upper echelons of the IRA.

Laidlaw is a Treasury Counsel, a government lawyer who has been employed to prosecute Britain’s first war crimes case, act as prosecuting lawyer against Al-Qaida suspects, as well as the IRA bombers who targeted Canary Wharf in 1996.

He has also acted as defence counsel for Brooks when she was acquitted of all charges in the phone-hacking trial.

Operation Kenova is headed by the chief constable of Bedfordshire, Jon Boutcher. He has promised to complete his report by the end of this year and will then hand his files to the Public Prosecution Service for Belfast.

Operation Kenova has now gathered more than 12,000 documents, recorded over 1,000 statements and conducted 129 with witnesses, some of whom alleged they were picked up, tortured, threatened and beaten by Stakeknife’s IRA unit.

As well as the agent himself, Operation Kenova is questioning members of the security forces over their role in running an agent they are accused of allowing to kill with impunity.

A spokesman for Operation Kenova declined to make any comment about Laidlaw’s appointment.