The government’s adviser on terror legislation has warned that there is “no magic test” to determine the risks posed by a terrorist on release from prison, in further criticism of recently announced proposals to examine them with lie detectors.

Jonathan Hall QC said in a speech that it was “impossible to guard against all risks” of violent reoffending and that any system which handed over release decisions to “risk experts” using polygraphs or any other method would be unacceptable.

Speaking to the Henry Jackson Society, the legal expert said that “there is no magic test for risk at the point of release” and said that terror sentencing decisions made by trial judges should not be minimised in favour of alternatives.

“There are also issues of principle here: a system in which length of sentence was entirely handed over to risk experts would be unacceptable,” Hall added.

Hall was partly referring to Usman Khan, a terror offender who killed two in a frenzied knife attack near London Bridge in 11 November, 11 months after he had been released on licence, prompting a political row over whether he should have been let out.

At the tail end of the election campaign, Boris Johnson, the prime minister, promised to implement a further crackdown on terrorism offenders. Full details were announced this week, dominated by the promise to introduce lie-detector testing.

Lie-detector tests remain inadmissible in UK courts because of their unreliability. Many experts say while they can detect physiological changes, they are not scientifically validated as reliable measures that someone is lying.

The Home Office also said it would insist that convicted terrorists serve their full term behind bars, although Hall warned that this pledge could only have a limited effect, given that those eligible for release had usually not committed violent acts.

“Many terrorist offences are what we call precursor crimes (for example possession of a document, or providing funds to a proscribed group),” Hall said, which mean that their jail terms would be limited.

“For this category of offenders the sentences will continue to be modest – so discussion of making terrorists serve their full sentences should not overlook the reality that many terrorists will be released sooner or later even if they do serve their full sentence.”

Hall called for more research to prevent a repeat of cases such as Khan’s, saying in his speech that “what makes an individual progress from terrorism in its broadest sense to acts of violence is poorly understood”.

MI5 and the counter-terror police should continue to evaluate and share information about convicted terrorists even after the end of formal investigations, Hall said, partly to see whether they re-engage in terrorist related activity.

A growing number of terror offenders have been released from prison in recent years, having committed offences during the era when al-Qaida was the leading group, but Hall warned that the UK authorities may also be faced with similar decisions if British Isis prisoners currently held in Syria are eventually released.

Hall, who was appointed last May, is given special access to secret national security information to carry out his task. Ministers appoint the independent reviewer to inform the public and political debate on anti-terrorism law in the UK.

Hall was also named earlier this week to carry out a review of the way the police, MI5 and probation service investigate, monitor and manage terrorist offenders.

The Ministry of Justice said there would be no role for polygraph testing in sentencing decisions made by trial judges in terror cases. Lie detectors are to be introduced to help determine when terror offenders should ultimately be released.