Teenagers convicted of terrorism in England and Wales could receive whole-life terms under sentencing reforms described by the government as the most radical in almost 20 years.
Currently, a life tariff can only be given to those over the age of 21, but ministers plan to reduce this to 18 for exceptional cases, such as terrorism.
A White Paper outlining further details will be published this week.
But Labour said the Tories had pushed the justice system “to the brink”.
The proposals will include whole-life sentences for those who kill children.
And there would be new powers to prevent the automatic release of offenders who have become radicalised behind bars while serving non-terror related sentences.
The planned reforms come after Hashem Abedi, who helped his brother Salman plan the Manchester Arena bombing, was jailed in August for at least 55 years.
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He was under the age of 21 at the time of the murders so a whole-life order was not an option open to the courts.
Sentencing Abedi, Mr Justice Jeremy Baker said a whole-life order would have been a “just sentence” in the “exceptional circumstances” but said he still “may never be released”.
Writing in the Sunday Express, Boris Johnson cited Abedi’s case, saying if someone plots to deliberately kill dozens of people “then it doesn’t matter if you’re ‘only’ 18, 19 or 20 when you do so”.
“We’re going to remove a loophole that lets some truly despicable criminals avoid such a sentence because they’re under 21 at the time of their crime,” Mr Johnson added.
“Dangerous criminals belong not on our streets but behind bars – with public protection the single most important principle of sentencing.”
Under the proposals, there will also be longer sentences for 15 to 17-year-olds who commit murder.
Justice Secretary Robert Buckland said: “From longer jail time for dangerous criminals to new measures to improve rehabilitation and cut reoffending – we are delivering a system that is more equipped than ever to crack down on crime, which the public can have confidence in to keep them safe.”
But shadow justice secretary David Lammy criticised the government, citing a figure in a Prison Reform Trust report which said there was a reoffending rate of 64% for those who spend fewer than 12 months in prison.
Mr Lammy said: “We will scrutinise this proposed legislation to make it as effective as possible, but we also ask ministers to hold their hands up for their mistakes that have pushed the justice system to the brink.”
A White Paper is expected to be published mid-week before legislation is laid before parliament in the new year.