The government wants to introduce emergency legislation to stop convicted terrorists from being released automatically halfway through their sentence.

It comes after two terrorists who were let out after serving half their time, without having to go before the Parole Board, carried out further attacks.

Sudesh Amman was shot dead by police in Streatham, south London, on Sunday after stabbing two people. Amman was given more than three years for a string of terror offences in 2018, but he was released at the halfway point.

In November last year Usman Khan killed two people near London Bridge. Khan was also released automatically halfway through his 16-year prison sentence.

Why is the change controversial?

Minsters now want terrorist offenders to be released after two thirds of their sentence at the earliest, and only after the independent Parole Board carries out a risk assessment.

This was the crucial bit in Mr Buckland’s speech: “We face an unprecedented situation of severe gravity and, as such, it demands that the Government responds immediately, and that this legislation will therefore also apply to serving prisoners.”

There is an ancient principle in English common law that criminals are sentenced in line with the laws or guidelines in force at the time they did the crime.

The idea is that it’s not fair to give people a harsher punishment than the one they might have expected to get at the time.

This explains why Rolf Harris got a relatively short prison term for abusing young girls between 1969 and 1986. The judge who sentenced him had to follow the guidelines that were in operation when he carried out the offences.

According to legal scholars, the idea behind this principle “can be traced in some form from Roman Law, through Magna Carta to the writings of Blackstone”.

The rule is also enshrined in Article 7 of the European Convention on Human Rights, which state: “Nor shall a heavier penalty be imposed than the one that was applicable at the time the criminal offence was committed.”

We know that Boris Johnson is aware of this principle because he brought it up in an interview with the BBC’s Andrew Marr following the London Bridge attack.

Mr Johnson said: “You cannot retrospectively change… you can’t change the basis on which someone is sentenced.” It appears that he has now changed his mind.

Did Labour bring in the rules that allowed the terrorists out halfway?

The short answer is yes – but there is more to it than that.

Labour’s Criminal Justice Act 2003 said prisoners serving determinate sentences would serve half in custody before being freed on licence.

In 2008, Labour changed the law again so that prisoners given extended sentences would be released automatically without going before the Parole Board.

But Labour also brought in something called an Indeterminate sentence for Public Protection (IPP), and this is the sentence that the London Bridge attacker Usman Khan was originally given.

The IPP sentence meant Khan would only have been released after a minimum term if the Parole Board was satisfied he was no longer a danger to the public.

But Khan’s original sentence was overturned on appeal and he was given an extended sentence by appeal judges. This meant he was eventually released without the involvement of the Parole Board – according to Labour’s 2008 rules.

So the case of Usman Khan is complicated. He was released on the Conservatives’ watch – but under rules brought in by Labour. But Labour also introduced a tougher kind of sentence, which Khan was in fact initially given, before judges overturned it.

Did the Conservatives “do nothing for ten years”?

That was the accusation from Andrew Marr in his interview with Boris Johnson in December last year.

In fact, Conservative ministers have made major changes to the way terrorists are sentenced over the past decade, though none of these changes kept Sudesh Amman or Usman Khan in prison.

Under the coalition, Conservative Justice Secretary Chris Grayling changed the law so that some “child rapists and terrorists” would no longer be automatically released halfway through their sentence.

The rules came into effect in 2015, and they followed the principle of only applying to future cases, not serving prisoners. They didn’t affect Usman Khan because he was sentenced in 2012.

And Mr Grayling’s changes only applied to people convicted of certain serious offences.

The Streatham attacker, Sudesh Amman, was convicted of less serious offences – dissemination of terrorist material and collection of information of a kind likely to be useful to a terrorist.

Those crimes weren’t not included in Mr Grayling’s 2015 rule change, so the change in the law had no effect on when Amman was released from prison.

Last year the Conservatives introduced more changes to the sentencing of terrorists: the Counter-Terrorism and Border Security Act 2019.

This went further than Chris Grayling and strengthened the rules on a long list of terror offences, including the ones Sudesh Amman admitted in 2018.

If someone were convicted of the same offences today, they would not now be automatically released halfway through their jail term.

But the Counter-Terrorism and Border Security Act came in 2019, after Amman was jailed. So, in accordance with that long-established legal principle, it did not apply retroactively.

FactCheck verdict

There is a well-established legal principle that criminals should not retrospectively be given a harsher sentence than the one in force when they carried out the crime.

Of course, legal scholars disagree about whether this is a good rule and what the consequences would be if parliament decides to break it.

There has been a lot of talk from politicians about who is to blame for the fact that some convicted terrorists are still being let out automatically after serving half their time.

As ever, the facts are complicated, and it’s worth noting that several different agencies were involved in the cases of Sudesh Amman and Usman Khan.

Police initially arrested Amman on suspicion of preparation of terrorist acts in 2018 – but he was charged with lesser offences on the advice of the Crown Prosecution Service.

Khan was originally given a sentence that meant he would have had to go before the parole board – but appeal judges changed the terms of his sentence so that he was eventually released automatically.

Both Labour and Conservative ministers brought in changes to the way terrorists were sentenced – but none of the policies stopped Khan or Amman being released halfway through their jail terms, or prevented them from re-offending.