Lie detector tests could be employed to monitor terrorists released from prison under licence in Northern Ireland, a watchdog has said.

A new law toughening sentencing of serious offenders proposed by the Government is passing through Westminster.

As well as a minimum 14 years in custody for the most dangerous it also envisages longer periods of state supervision after release from jail.

The proposals would empower Stormont ministers to include the lie detector, known as a polygraph, in the release licences of certain terrorist offenders.

Terrorism legislation reviewer Jonathan Hall QC said: “Use of polygraphs for the purpose of administering licences requires clear and public guidance as to the use to which testing is to be put, and careful thought given to when a polygraph condition should be included.

“This is no more so than in Northern Ireland, where a distinction will need to be drawn between permitted factual questions for the purpose of ensuring compliance with licence conditions, and (what would not be permitted) general intelligence-gathering on associates.”

Polygraphs are not currently used for offenders in Northern Ireland, which is different from England and Wales where they are employed to assist in monitoring sex offenders on licence.

The challenges posed in monitoring terrorist offenders are such that polygraph testing is a sensible additional tool

If the Counter-Terrorism and Sentencing Bill, designed to standardise measures across the UK and requiring the assent of the Stormont Assembly, is passed and agreed to by Stormont, Northern Ireland would need to develop a polygraph infrastructure.

Mr Hall said: “The challenges posed in monitoring terrorist offenders are such that polygraph testing is a sensible additional tool.

“The amendments (in the draft legislation) would not require but empower Scottish ministers, and the Department of Justice in Northern Ireland, to include polygraph conditions in the licences of certain terrorist offenders.”

The Government wants the new legislation to apply UK-wide to ensure equal provision of counter-terrorist measures.

Stormont will have to give its consent for some aspects of the changes.

These new sentences carrying a minimum 14-year custodial term are intended to apply to dangerous offenders convicted of terrorism or terrorism-connected offences carrying a maximum of life imprisonment where the offence was very likely to cause or contribute to the deaths of at least two people.

Such sentences must be passed unless the offender is sentenced to life imprisonment or an indeterminate custodial sentence, the reviewer said.

Where those types of indeterminate sentences are passed, a minimum 14-year custodial term must be imposed.

Sentences for terrorism offences are generally lower in Northern Ireland than the rest of the UK.

Mr Hall said aspects of sentencing called into question whether serious terrorism sentences will be passed as frequently as they might, were the same conduct to occur in Great Britain.

The law would require the sentencing judge to make a determination of dangerousness but the Probation Service has historically not carried out such risk assessments since it cannot access intelligence material.