The killer of eight-year-old Sarah Payne will spend 50 years in jail despite a landmark legal ruling today.

Home Secretary David Blunkett's power to set minimum sentences for murderers was today ruled incompatible with human rights law.

The ruling by the Law Lords was the first step towards stripping home secretaries of that power.

Mr Blunkett yesterday announced sex killer Roy Whiting would be jailed for half a century for snatching Sarah from a cornfield in Kingston Gorse, East Preston, and murdering her.

In a test case ruling affecting 225 prisoners, a panel of seven Law Lords upheld arguments by defence lawyers that the task of fixing minimum tariffs for convicted murderers should be handed back to judges.

Lawyers had argued the Home Secretary had a duty under the Human Rights Act not to violate a prisoner's right to a fair hearing.

Edward Fitzgerald QC told the Lords the setting of tariffs was "a decision that should be taken openly, publicly and fairly in court by a judge, not secretly and unfairly by a politician who has never heard the case and is subject to all the inevitable pressures of public opinion."

Around 70 lifers have already served more time than originally recommended by the judiciary and could now be freed immediately.

But Whiting, 43, from Littlehampton, will not be among them because the judge at his trial ruled he should never be released.

The decision effectively means Whiting will die in jail.

The decision to keep him locked up for another 50 years was welcomed by Sarah's mother, Sara, who feared he could be freed within 20 years to prey on her grandchildren.

"We can finally step out of his shadow now," she said. "When we told the children the news there were big smiles from the boys."

The minimum sentences for some 225 murderers which were set by the Home Secretary may have to be reviewed as a result of today's historic ruling.

Of those, 70 have already served more than the minimum originally recommended by their trial judges.

According to some campaigners, the Law Lords' decision means that they could be freed within months.

Cases in which trial judges have recommended minimum terms but the Home Secretary has decided that "life means life" include some of the most infamous criminals alive in Britain today, including Rose West and Dennis Nilsen.

Their situations could now be reviewed.