A former Oxford University research chemist has been jailed indefinitely for sexually assaulting a toddler and building up a "vast" child porn library.

Andrew Lintern shared dozens of the illicit images with fellow paedophiles and collected a large number of “disturbing” stories.

Southwark Crown Court in London heard that as his dangerous "obsession" with children grew, he eventually assumed the identity of his former victim in an online bid to ensnare another youngster.

Pretending he was just nine, the childless 55-year-old tried to convince what he thought was a schoolgirl to meet him for sex.

But officers from the Metropolitan Police's Paedophile Unit were waiting for him instead.

Prosecuting, Hannah Llewellyn-Waters said when officers raided his home in Kings Langley, Hertfordshire, they recovered nearly 20,000 child sex images and video clips he had spent years collecting.

After his arrest he discovered the 13-year-old he thought he had spent months talking to was in fact a police officer.

Judge Geoffrey Rivlin said Lintern would have to serve a minimum of three-and-a-half years in prison before being considered for parole.