Sarah Payne's killer is to appeal against his 50-year jail sentence.

Paedophile Roy Whiting, 47, claims the tariff is unlawful and if successful he could see his life sentence cut by almost half.

Sara Payne, whose eight-year-old daughter was snatched and killed on a day out in Sussex in 2000, is said to be "shocked and distraught" by the news.

Pressure group Phoenix Survivors, which acts for victims of child sexual abuse, said it will join Sara, who is a member, to fight Whiting's challenge.

A spokeswoman said: "We will do everything in our collective powers to prevent Roy Whiting from taking a single breath of freedom - ever again.

"How can the system do this to Sara and her family? What are they thinking? Hasn't this family been through enough?"

Whiting was given one of the longest tariffs imposed in the history of the British legal system when David Blunkett ruled he should serve at least 50 years.

The appeal alleges Mr Blunkett was driven politically and not solely by judicial motives in trying to ensure Whiting would never be allowed back into society.

The case has been lodged at London's Administrative Court by leading law firm Irwin Mitchell.

Sarah's grandfather Terry Payne said he was stunned. Whiting admitted two years ago kidnapping and killing Sarah as she played in cornfields by her grandparents' home in Kingston Gorse, near Littlehampton.

Mr Payne said people like Whiting did not deserve the chance of freedom.

He said: "They let these guys out of prison without any supervision. 'There you go sunshine, away you go,' they say.

"It has happened before and it will happen again. These guys have to be put away."

Whiting was jailed at Lewes Crown Court before the tariff was set but Mr Blunkett rejected the Lord Chief Justice's recommendation of 28 years when he sentenced the killer.

A year after Whiting was convicted it was revealed Mr Blunkett had ordered he must serve 50 years in prison.

The following day, November 25, 2002, Law Lords ruled the Home Secretary be stripped of his powers to impose such sentences as they were not compatible with the European Convention on Human Rights.

Lawyers will challenge Home Office declarations that the timing of Mr Blunkett's announcement was coincidental.

The Home Office said yesterday: "The then Home Secretary made the decision, within his powers at the time, based solely on the evidence presented to him."