The landmark ruling could have far-reaching effects on other cases, including that of Moors murderer Myra Hindley, who is preparing a court challenge against a "life must mean life" sentence imposed by Mr Howard.
The House of Lords ruled by a 3-2 majority that Mr Howard had wrongly increased from 15 to 20 years the "tariff" - the minimum period to be served for the purposes of punishment and deterrence - by double murderer John Pierson.
Pierson shot his sleeping mother and father at their farmhouse near Oswestry, Shropshire, in September 1984, receiving a double life sentence for what were described as "horrifying and apparently motiveless" killings.
The judge at his trial in July 1985 and the then Lord Chief Justice Lord Lane recommended that his tariff should be 15 years. He was told in August 1993 that the Home Secretary had increased the minimum to 20 years.
But Lord Hope of Craighead ruled: "The Home Secretary does not have a general power to increase the period which he or his predecessor has fixed as the minimum ... once the decision has been issued and communicated." That did not mean that the minimum might never be increased where "exceptional circumstances" made this necessary, Lord Hope said. But this was not such a case.
Lord Steyn and Lord Goff, for different reasons, agreed but Lord Browne- Wilkinson and Lord Lloyd of Berwick gave dissenting judgments.
Jack Straw, the Home Secretary, declined to comment on the implications for Myra Hindley, but said that the ruling did not directly affect the date on which an individual, having served the period of the tariff, is released or whether they should ever be released.
A spokesperson for the Home Office said later that Mr Straw would exercise his discretionary powers of release only if he could be certain that it would not undermine confidence in the criminal justice system.
Hindley's tariff, unknown to her, was originally set at 30 years by the then Home Secretary, Leon Brittan, in 1985, but by 1990 another Tory Home Secretary, David Waddington, had revised it upwards to whole life. After a House of Lords ruling in 1994 enabling prisoners to be told their tariffs, she made representations, but earlier this year her tariff was again reset at whole life by Mr Howard. That decision is scheduled to be judicially reviewed later this year.
About 20 other cases where tariffs have been subsequently increased could be affected by yesterday's ruling. Among those told they would never be released were Jeremy Bamber, who was convicted in 1986 of murdering his adoptive parents, his step-sister and her six-year-old twin sons.
John Wadham, director of the civil rights group Liberty, said: "The only solution for the future is for courts and not politicians to decide how long people spend in prison."
Kate Akester, of the law reform pressure group Justice, said: "The ruling means that other cases where tariffs have been raised by the Home Secretary will obviously have to be looked at immediately. It also means that it's another step towards taking the Home Secretary out of the tariff-setting hoop."
Brian Mawhinney, shadow Home Secretary, said it was now up to the Government to change the law.
"The judges' decision puts great pressure on the Home Secretary not to take refuge in platitudes but to see that the law is amended so that people who commit such murders receive tougher sentences."
The Rev Peter Timms, a methodist minister and former prison governor who has counselled Hindley, said the ruling "may give the Home Office the chance to think again about the whole matter, which I hope they will. To keep adding to the tariff the way they have done with her and one or two more is quite vindictive."Reuse content