At least 500 victims, some as young as two years old, were abused by the shamed television presenter Jimmy Savile, new research shows.

A study by the National Society for the Prevention Cruelty to Children in Britain, commissioned for BBC Panorama, reveals the extent of Savile's offending and his access to Broadmoor hospital, where some of his abuse took place.

The report said the scale of Savile's offending inside Broadmoor is higher than previously thought, with Thames Valley Police having received 16 reports of abuse by him inside the special hospital.

It also describes how some awe-struck civil servants erroneously referred to the Top Of The Pops presenter as "doctor", unaware of the trauma he was inflicting on some youngsters behind hospital doors.

The figures show the most common age group for Savile's victims was 13 to 15 - and the youngest alleged victim was just two years old.

Peter Watt, the NSPCC's director of child protection, said: "There's no doubt that Savile is one of the most, if not the most, prolific sex offender that we at the NSPCC have ever come across.

"What you have is somebody who at his most prolific lost no opportunity to identify vulnerable victims and abuse them."

The joint BBC investigation between Panorama and The World At One, which airs today on BBC One and BBC Radio 4, asks how the DJ got so close to the heart of Britain's establishment and why in 1972 the BBC failed to take effective action that might have saved young people from abuse.

In 1988, health minister Edwina Currie appointed Savile to head a task force to address tensions between Broadmoor's management and unions.

A confidential Department of Health memo obtained by Panorama suggests that his appointment was being pushed by a senior civil servant.

The documents suggest civil servants were impressed by the entertainer, who died in October 2011. They refer to him as "doctor" Savile and say he wanted to "ease out" staff at the hospital.

They say he went through each of the main departments at the hospital like a "dose of salts".

Savile first became involved with Broadmoor through the League Of Friends charity in the late 1960s. He was later given his own set of keys and a house in the grounds.