LONDON (Reuters) - "Yorkshire Ripper" Peter Sutcliffe has begun a legal challenge to win freedom, on the grounds that the government failed to set the exact length of his sentence, his lawyers said on Wednesday.

Sutcliffe, 61, was jailed for life in 1981 for the murders of 13 women over five years in one of Britain's most notorious criminal cases.

His London law firm Bindmans said the state has a legal obligation to set a tariff, the minimum term that life sentence prisoners must serve before becoming eligible for parole.

"Any prisoner is entitled to have a tariff set within a reasonable time of conviction which will set out the minimum term of imprisonment to be served," it said in a statement.

Media reports said Sutcliffe wants to be moved back into the prison system from the Broadmoor secure hospital in Berkshire, where he has been treated for mental illness.

If he is returned to prison, his condition would be reassessed and the length of his sentence formally laid down, the BBC said.

His solicitor Saimo Chahal and barrister Paul Bowen would make no comment beyond their prepared statement.

It said an unnamed court was due to consider the case of "Peter Coonan", the name Sutcliffe now uses, under the terms of the Criminal Justice Act 2003.

The court will set a tariff in due course, the statement added, without giving further details.

A Ministry of Justice spokesman said: "We do not comment on individuals. Sentencing is a matter for the courts."

A Home Office spokeswoman declined to comment.

Home Secretary Jacqui Smith told the BBC News website: "Top of my list of priorities, I have to say, is not Peter Sutcliffe's rights, it's the rights of those people who were his victims, and how we keep this country safe."

Sutcliffe preyed on lone women, some of them prostitutes, in Bradford and Leeds in a five-year reign of terror.

He gained the name the Yorkshire Ripper because of the way he mutilated his victims after murdering them, as did the original Jack the Ripper in east London 120 years ago.

At least he got some kind of punishment Ian Kay.

Screw the prision sentence. Just have him shot. Waste of my tax payers money.

Hang the bastard, let him choke to death on the end of a rope.

Sick cunt, let him rot in prison.