Ian Huntley has been given a mandatory life sentence but the tariff - the minimum term he actually serves - has yet to be decided.
Huntley murdered 10-year-olds Holly Wells and Jessica Chapman
Until a year ago it would have been down to Home Secretary David Blunkett to decide the length the murderer stayed in prison.
But the system of tariff-setting is in the process of being changed.
From Thursday offences such as Huntley's will carry whole life terms.
Because Huntley's offence was committed in August 2002, his case is subject to transitional arrangements.
This means the minimum term that he will serve in prison will be decided by an appeal court judge - to be appointed by the judiciary - not the trial judge.
'Whole life' tariffs
The Home Office says this tariff-setting process will take months.
The judge must take into account the new principles set out under the Criminal Justice Act, which became law on Thursday.
The new principles state that "whole life" prison terms are given for:
Multiple murders - two or more - that show a "high degree of premeditation, involve abduction of the victim or are sexual or sadistic".
Murder of a child following abduction or that involves sexual or sadistic conduct. The principles also state that there will be a starting point of 30 years for other multiple murders.
Mr Blunkett said that under the old system, the last time he had a similar case - Roy Whiting, who abducted and murdered Sarah Payne - he imposed a 50 year tariff.
Although he can no longer decide whether life means life Mr Blunkett told journalists he hoped Huntley's sentence would reflect the fact that under the old system - and the one which begins shortly - a 50 year or "whole life" tariff would be expected.
Later, criminal justice sources suggested it was "theoretically possible" that Huntley could get a "whole life" tariff.
Tariffs like this had been set by successive home secretaries in exceptional cases in the past, such as for Moors murderer Ian Brady.
"It follows that having regard to that and to the details of this case, it is theoretically possible that an Appeal Court judge might think that appropriate," the source said.