A senior officer who investigated the Army’s top agent within the IRA has urged extreme caution over closing down inquiries into unsolved murders in Northern Ireland.

Jon Boutcher led Operation Kenova, probing the activities of the mole codenamed Stakeknife, and said it showed that opportunities for prosecution could be identified.

In March, Northern Ireland Secretary Brandon Lewis announced that only Troubles killings with “compelling” new evidence and a realistic prospect of court proceedings would receive a full re-investigation by police.

The Government said that, after a review, most unsolved cases would be closed and a new law would prevent those investigations from being reopened.

Mr Boutcher said: “The closing of investigations of unsolved murders so that they cannot be reopened would be a new legal stance.

“The proposal to close down investigations of murder in legacy cases after a quick review process where those cases could not be reopened would, I believe, be a legal novelty in the United Kingdom for serious crimes such as murder.

“In light of the opportunities identified by Operation Kenova, this proposal should be approached with extreme caution, especially as regards the processes applied to establish what information exists about those cases.”

It should never be the case that those responsible for crimes such as murder are protected by a lack of a thorough examination of the facts

As part of the independent Operation Kenova, four people, including two former MI5 officers and a former prosecutor, will not face criminal proceedings as part of a major investigation. Other prosecutorial decisions are outstanding.

Mr Boutcher added: “An investigation/review which starts and finishes only with the information available at the outset and does not allow for the development of lines of inquiry would not be Article 2 ECHR-compliant.”

Article 2 of the Human Rights Act surrounds the protection of life.

Stakeknife is alleged to have been west Belfast man Freddie Scappaticci, 73, who was arrested for questioning in the course of the investigation.

He has denied being the Army agent, who worked within the IRA’s notorious “nutting squad” interrogating suspected informers during the Troubles.

Mr Boutcher added: “It should never be the case that those responsible for crimes such as murder are protected by a lack of a thorough examination of the facts.

“Prosecutions are exceedingly challenging in legacy cases and I would expect them to be very much the exception.

“The starting point for legacy should be finding the truth for families of what happened.

“Families want to be listened to, acknowledged and for an investigation to take place that is an independent and robust search for the truth.

“They are generally realistic about the scope for seeing culprits brought to justice and punished and about the practical utility of such an exercise at this point in time.”