A PROLIFIC paedophile who used the internet to target and blackmail young girls all over the world, including a girl in Tasmania who was key to his arrest, has been jailed in a case described as one of the worst British courts have ever seen.

The "sadistic exploitation" conducted by Robert Hunter, of Middlesbrough, was so extensive that an international operation through Interpol was sparked.

This came after the 35-year-old had used sites such as Facebook, Skype and MSN to abuse girls as young as nine from across Europe, Asia, Canada and the US.

He would also often pose as the singer Justin Bieber in an attempt to convince the girls to expose themselves and perform sex acts on a webcam.

Teesside Crown Court heard that after being arrested in December 2012, he continued to target children and this resulted in a girl from Tasmania eventually contacting police, who traced him back to his home.

In total, more than 800 videos and images were discovered on his computer, all of them made by the defendant.

Richard Bennett, prosecuting, said over the course of 2010, 2011 and 2012, Hunter had pretended to a number of very young girls that he was a teenager using a number of online aliases.

"On each occasion he was able to disguise his true age and identity by the clever use of images of young boys or by pretending that his computer wasn't working properly.

"As a further demonstration of the naivete and innocence of his victims, he was also able to persuade some that he was the music artist Justin Bieber.

"He did this in order to dupe and encourage these young girls to strip on webcams and perform sex acts for him."

Hunter also blackmailed some of the victims into agreeing to his demands by threatening to make the videos he had already made of them public.

Sentencing Hunter to 14 years in prison, Judge Peter Bowers said it represented a callous and sadistic exploitation.

"This is one of the most serious cases on internet abuse that the courts have dealt with," he said.

"It's a warning to all parents of teenage children of what can be done via the internet.

"The public will be disgusted by how you have behaved even after you had been arrested and bailed.

"It represents callous and sadistic exploitation of a number of girls over a number of years."

Tamara Pawson, defending, said Hunter was remorseful for his actions and the distress he had caused.

In total, Hunter pleaded guilty to 30 offences, which included 15 charges of causing or inciting a child to engage in sexual activity and 14 of making indecent photos of children.

Sergeant Paul Higgins, from Cleveland Police, said Robert Hunter was a calculating and malicious predator who set out to coerce and bully young girls into exposing themselves over the internet.

"This investigation highlights the dangers that children face when conversing with people over the internet," he said.