At least 500 girls and boys - some as young as two - were abused by disgraced television presenter Jimmy Savile making him one of the UK's most prolific sex offenders, new research has found.

A study by the NSPCC, commissioned for BBC Panorama, reveals confidential documents that shed light on the extent of Savile's offending and his unprecedented 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 and found the most common age group for Savile's victims was 13 to 15 – with the youngest alleged victim being 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 on Monday 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.

It comes ahead of the publication of BBC's own review to be carried out by Dame Janet Smith. That report, whose publication has been repeatedly delayed, is expected to uncover up to a thousand victims and reveal a culture of ignorance which protected Savile.

A BBC spokesman said: "The BBC has said it is appalled at Jimmy Savile's crimes. We're unable to give a commentary on statements in these specific documents as they are over 40 years old, but the Dame Janet Smith review is considering the culture and practices of the BBC during this period and has had our full cooperation. We will await their full report."