Leading judges will give a crucial ruling on whole-life sentences today.
A panel of five judges, headed by the Lord Chief Justice Lord Thomas, will announce their decision on controversial "life-means-life" orders at the Court of Appeal in London.
Sentencing in a number of high-profile criminal cases has been put on hold - including the terms to be handed out to soldier Lee Rigby's murderers - pending today's judgement.
At a hearing in January the judges were urged to find that imposing sentences which mean a prisoner can never be released are not "manifestly excessive or wrong in principle".
The Government has said that whole-life tariffs are "wholly justified in the most heinous cases".
Such terms were deemed a breach of human rights following a successful appeal to the European Court of Human Rights (ECHR) by murderers Jeremy Bamber, Douglas Vinter and Peter Moore.
Last year the trio won a ruling that their whole-life sentences amount to "inhuman and degrading treatment".
Whole-lifers should be entitled to a review of their sentence 25 years into their term at the very latest, the Grand Chamber of the Strasbourg-based court said.
The ruling by 17 judges from across Europe sparked further outrage among critics of the court - despite reassurances that the decision did not amount to grounds for imminent release.
As well as the spotlight being on whole-life orders generally, the Court of Appeal judges heard argument on behalf of Lee Newell, who murdered child killer Subhan Anwar in prison, in his challenge against his whole-life order.
They will give their decision on the Newell case today along with a ruling on a case referred to the court by Attorney General Dominic Grieve.
The judges have been asked to rule on whether a tariff of 40 years imposed in the case of Ian McLoughlin for murdering a man when on day release can be regarded as "unduly lenient" and should be increased.
When making submissions on whole-life tarrifs, James Eadie QC, representing Mr Grieve, told the judges that if a sentencing court "is of the opinion that, because of the seriousness of the offending, a minimum term should not be fixed, the court must impose a whole-life order".
He submitted: "The failure to impose a whole-life order in circumstances where the seriousness of the offending requires its imposition renders the sentence imposed unduly lenient".
Mr Eadie said the decision in Europe "does not render the imposition of a whole-life order manifestly excessive or wrong in principle".
In the case of McLoughlin, the judges heard that he was aware of the proceedings but did not wish for any argument or representations to be made on his behalf.
His stance was explained to the court by barrister Kevin McCartney, who said McLoughlin had not considered the legal aspect, but had approached it from a "purely personal approach".
It appeared from letters written by McLoughlin that he had been "very anxious at the sentencing hearing" and that this was a "sentiment that carried on... not to act in any way that would cause any further distress to the deceased's family".
Mr McCartney said: "That is a factor that played very heavily, as I understand it, in his attitude towards these proceedings."
It was argued on behalf of the Attorney General that the "failure to impose a whole-life order renders the sentence unduly lenient".
On behalf of Newell, Joe Stone QC, in seeking permission to appeal against sentence, said that a whole-life term was "manifestly excessive".
If he was given a 40-year minimum it would give him hope and the "flickering possibility that one day when he is 85 he will be released from prison".
Newell, now 45, is challenging a whole-life sentence imposed last September at Warwick Crown Court.
He was convicted alongside Gary Smith for the February 2013 murder of convicted child killer Anwar in his cell at Long Lartin Prison, Worcestershire. Newell was already serving a life sentence for a previous murder committed in 1988.
Triple killer McLoughlin, 55, was jailed for life at the Old Bailey last October for stabbing a man on his first day-release from prison after 21 years in custody.
When sentencing McLoughlin, the trial judge imposed a 40-year tariff, saying he could not pass a whole-life term because of the European court ruling.
McLoughlin - who had killed twice before - stabbed Graham Buck, 66, as he came to the aid of a neighbour in Little Gaddesden, Hertfordshire, last July.
Those currently serving whole-life terms in England and Wales include Moors Murderer Ian Brady, who tortured and murdered children along with accomplice Myra Hindley, and serial killer Rosemary West.
Today's ruling will be given by Lord Thomas, Sir Brian Leveson, Lady Justice Hallett, Lord Justice Treacy and Mr Justice Burnett.
Mr Justice Sweeney has said that he will wait to sentence Lee Rigby's killers, Michael Adebolajo, and Michael Adebowale, until after the Court of Appeal decision. No date has yet been fixed for that hearing.