Peter Sutcliffe, the mass murderer dubbed the Yorkshire Ripper because he mutilated the bodies of his female victims, has begun a legal challenge to win freedom.

His lawyers said today they were bringing an action to establish the exact length of his sentence, as the first of a series of legal moves allegedly intended to have the killer free by 2011.

Sutcliffe, 61, was jailed for life in 1981 for murdering 13 women and attempting to kill seven more over a five year reign of terror in the north of England, in what became one of Britain’s most notorious criminal cases. He was sentenced to 20 life sentences, and told he must serve a minimum of 30 years behind bars.

Sutcliffe has however spent the majority of his 27 years in captivity in a secure mental hospital. It emerged in 2006 that he is not one of the 35 'worst' offenders who have been told that they must spend their entire life behind bars.

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.

A statement from Bindmans said today: “This firm is instructed by Mr Coonan (previously known as Peter Sutcliffe) in respect of the setting of a minimum tariff following his conviction.

“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.

“Following a reference to the Court by the Secretary of State for Justice in accordance with the terms of the Criminal Justice Act 2003, the Court will consider the evidence and set a tariff in due course.”

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

Sutcliffe began his sentence in prison but three years later was diagnosed with schizophrenia and was transferred to Broadmoor secure hospital in Berkshire for treatment.

Media reports said Sutcliffe wants to be moved back into the prison system. Ms Chahal will argue his case for freedom in stages, with her first aim to get Sutcliffe back into the prison system, according to The Sun.

The lawyer has requested a reassessment of his psychiatric condition and aims to secure his release by 2011, reports said.

Times Recommends

• Is global warming the new apocalypse?
• Great novel? You must be having a laugh
• Haiti: the aid army marches on its stomach