The government will try to pass emergency laws by February 27 to stop any more terror offenders being released from jail early, in the wake of the Streatham and London Bridge attacks.
It is understood that unless MPs and peers pass potentially controversial new legislation to end automatic early release by that date, one terror offender would be released on February 28.
Five more will be released in March if the laws are not passed in time.
A Whitehall official said: “If the legislation is passed by February 27 we can prevent the automatic release of any further terrorist suspects who might pose a threat to the public.”
Three people were injured when 20-year-old Sudesh Amman began attacking members of the public with a knife in Streatham High Street on Sunday.
Amman, 20, was handed a three-year and four-month sentence in December 2018 for possessing and distributing terrorist documents.
He had recently been freed from jail and was under 24-hour surveillance by police, having served half of his sentence under the early release scheme.
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The incident had echoes of the London Bridge attack last year in which Usman Khan killed two people having previously been released on licence halfway through a 16-year sentence for his part in planning terror attacks.
The Whitehall official said the attack in Streatham had highlighted an issue surrounding terrorists with relatively short prison sentences.
“There aren’t many terrorist offenders who will be in that similar kind of scenario but if there are any then that’s too many and that’s what we are looking to fix,” the official said.
On Monday, Justice Secretary Robert Buckland said the urgent legislation was needed to make sure offenders serve two thirds of their sentence before they are considered eligible for release, at which point their case would be considered by a panel of specialist judges and psychiatrists at the Parole Board.
There are 224 terrorists in prison in Britain, with most thought to be holding Islamist extremist views, according to the latest published figures to the end of September.
As many as 50 terrorists could be freed from jail this year, figures suggest.
it is believed that the government may eventually look to set up a system where terror offenders can only be released if they are judged not to be a threat to the public.
But it must avoid rehashing indeterminate sentences for public protection (IPPs) which were scrapped in 2012 following legal challenges.
The government plans to introduce the legislation in the Commons on Tuesday next week, with the aim of clearing the House by the time it rises for recess on Thursday.
The bill will then go to the Lords on February 25 with the aim of getting royal assent on February 27.
The official said the Lords should “wish to carry out its scrutiny quickly” as “we cannot continue to be in a position where the state has no power to block the release of terrorists who continue to pose a threat to the public”.
HuffPost UK understands that lawyers in the Lords may have concerns about the rapid timetable, but peers have not held up emergency legislation for years.
The government has not ruled out derogating, effectively suspending, the European Convention on Human Rights (ECHR) in order to apply the new measures.
Lawyers have warned the move would open the government up to legal challenges from those already behind bars who were sentenced under the current rules.
But officials said they are confident they have the flexibility to change how an offender serves their sentence, by extending the time they spend behind bars rather than on licence.
It came after Metropolitan Police commissioner Cressida Dick said it was “inevitable” that there would was a short delay before officers trailing Amman responded to his stabbing attack in Streatham.
She told the London Assembly Police and Crime Committee: “They [officers] are conducting covert surveillance, so they are not of course providing man-to-man marking. They are there covertly and that is a deliberate thing.
“It is inevitable that there could be a time delay before somebody totally unexpectedly does something.”
She added: “I wish I could assure the public that everybody who poses a risk on the streets could be subject to some sort of thing that would stop them being able to stab anybody ever, but it is clearly not possible.”