The former England footballer Adam Johnson has been jailed for six years for sexually assaulting a 15-year-old schoolgirl in a “calculated, considered and carefully orchestrated” abuse of his celebrity status.

Johnson, 28, showed no emotion as he was imprisoned for kissing and sexually touching the teenage girl, whom prosecutors said he groomed in a “single-minded pursuit of his own sexual gratification”.

Johnson, who has been stripped of his 12 England caps, admitted kissing and grooming the besotted fan on the first day of his trial and was later found guilty of a more serious child sexual assault.

Sentencing him, the judge, Jonathan Rose, said he had abused his position of trust with a girl 12 years his junior in a way that had caused her “severe psychological harm”.

It can now be disclosed that Johnson was also arrested for possessing extreme pornography – believed to be a bestiality video – on his laptop, after the judge lifted reporting restrictions.

Johnson was arrested for possessing the video after detectives searched his laptop following his arrest over child sex offences on 2 March 2015. Police are understood to have have told Johnson he will face no further action over the video, which is said to have been “jokey” in nature.

Victim ‘very intimidated by it all’

In an impact statement read to court, Johnson’s victim said she had “entered many dark places” since she reported the assaults to the police after confiding in her parents.

Even after Johnson was found guilty earlier this month, she said, she had still been bullied at school, causing her to miss classes and her schoolwork to suffer “massively”.

“The whole experience has been very overwhelming and throughout the process I’ve had many hard times. I’ve always felt at risk of being recognised when I go places and on some occasions this has happened,” she said.

“I have felt very intimidated by it all and I’ve felt very lonely at times when I’ve felt everyone is against me due to Adam Johnson protesting his innocence and continuing to put himself in the public eye, and ultimately it was like I was being taunted as if to say he could do what he wants and get away with it.”

In a separate victim impact statement, the girl’s mother said her family had endured “heartache, criticism and slanderous remarks” and that she felt powerless to protect her daughter who, she said, had been subjected to “thousands of malicious remarks and some disturbing threats of violence made towards her”.

Johnson had denied all four allegations until the first day of his trial, when he pleaded guilty to one count of grooming and one count of kissing the schoolgirl. He continued to deny that he did anything other than kiss the teenager and disputed her two allegations of sexual offences.

Dave and Sonia Johnson, the parents of Adam Johnson, leave Bradford crown court. Photograph: Oli Scarff/AFP/Getty Images

But on 2 March he was found guilty by a majority verdict of carrying out a sex act on her in his black Range Rover behind a Chinese takeaway in County Durham on 30 January 2015.

Chewing gum in the dock, Johnson was told he would be on the sex offender register for the rest of his life and undertake treatment for what one psychologist described as his “hypersexual disorder”.

His parents, Sonia and Dave, showed no emotion as he was jailed for six years. He was also ordered to pay £50,000 towards the prosecution’s fees – amounting to less than a month’s pay on his now-terminated Sunderland football club contract.

Sentencing Johnson, Rose told the former Sunderland and Manchester City winger that he took advantage of “a young teenager’s adoration of a successful celebrity”.

‘Deliberate decision to engage in sexual activity’

The judge added: “You made a deliberate decision to engage in sexual activity with this young girl and to ignore the fact that she was only 15, no doubt in the hope and expectation that you would get away with it because both she and you would delete all WhatsApp messages referring to your contact and because you thought you could rely on her to tell no one, just as you had asked.”

It was revealed in court that Johnson told a psychologist days before sentencing that he viewed his victim “like any of the girls I met”. He told a doctor: “I put her age out of my mind. I was sexually interested but she was just another girl, another opportunity. She was attractive enough. Another one to get with.”

Psychologist Dr Philip Hopley, a deputy director of the Priory clinic in London who interviewed Johnson, said the footballer had expressed genuine remorse for his actions.

However, he appeared to show little contrition before his sentencing – at one point smirking and winking to a friend in the public gallery, chewing gum in the dock and earlier running into court in an attempt to evade the photographers.

Each of the psychologists that examined Johnson concluded that he needed treatment but that he did not pose a significant risk to children. The medics described Johnson as having a very high libido and a tendency to engage in sexual activity to a compulsive degree with a number of women.

Gerry Wareham, CPS chief crown prosecutor for the north-east, reads a statement outside Bradford Crown Court. Photograph: Peter Byrne/PA

Prosecutor Kate Blackwell QC said Johnson’s offending was “calculated, considered, carefully orchestrated” and that he still represented a threat to young women, and that he “was in the habit of meeting girls on the way back from training to have sex” in clandestine locations.

Blackwell urged the judge to consider as an aggravating factor that those close to Johnson – including his sister, Faye Johnson – had “openly encouraged and promoted” his supporters to “repeatedly and doggedly” abuse the footballer’s victim on social media.

Faye Johnson is behind a Facebook page with nearly 5,000 supporters of the footballer, some of whom have used the page to attack the victim.

After sentencing Johnson, the judge lifted reporting restrictions that prevented disclosure of a number of details. It can now be reported that Johnson was taking medication that can be used to treat sexually transmitted infections. Detectives discovered the medication in a safe while searching his £1.7m mansion in Castle Eden, County Durham.

Speaking outside court, Gerry Wareham, CPS chief crown prosecutor for the north-east, said: “Adam Johnson exploited a young star-struck fan, actively grooming her over a number of months in single-minded pursuit of his own sexual gratification. He gave no thought to this girl’s interests or welfare during that time or at any point since, repeatedly denying any wrongdoing for nearly 11 months until his trial began.

“We should all be clear that there is only one victim in this case: the 15-year-old girl he sexually assaulted.

“This situation had a devastating effect on the young victim and it is my sincere hope that she and her family will be granted the respect and space they deserve, to recover and rebuild their lives.”

Johnson’s barrister, Orlando Pownall QC, said they would appeal against the guilty verdict.