Christopher Sallon QC, chairman of the public affairs committee of the Bar Council, said: "I don't know of any other cases. It is very unusual."
The decision to grant early release in England and Wales rests with the Home Secretary, but in Clegg's case it was in the hands of the Secretary of State for Northern Ireland, Sir Patrick Mayhew, because the offence took place in the province.
The question they have to answer is whether the murderer has "served a sufficient period in custody, including when on remand, to meet the needs of retribution and deterrence for the offence", and if he or she is likely to be a risk to the public.
Judges can make a recommendation to the minister when they sentence a murderer to life. But although this may be for only five or six years, the ultimate discretion lies with the politician, who is usually under the opposite pressure from the Clegg case, and if anything will keep people in prison for longer than the judges have recommended.
Michael Howard, the Home Secretary, extended the judges' recommendations to 15 years for the two boys who killed the toddler, James Bulger.
Clegg's case has added to pressures for an overhaul of the law on murder.
Although his appeal failed, the Law Lords questioned the way judges have to set a mandatory life sentence for murder. Both the Lord Chief Justice, Lord Taylor of Gosforth, and his predecessor, Lord Lane, have called for the scrapping of mandatory life sentences.Reuse content