A maths teacher has been jailed after having a sexual relationship with a pupil while in his first job - at a top state school which David Cameron's eldest daughter goes to.

Christopher Stephenson groomed the teenage girl over Facebook before embarking on an 18-month relationship with her while she was still a pupil at the prestigious Grey Coat Hospital School in Westminster, where he taught.

He took her for day trips, got her drunk on a night-out in London and once took a photograph of her topless in his bed, and on a number of occasions they carried out sex acts on each other.

The 32-year-old's teaching career was left in tatters after he was jailed at Southwark Crown Court on Monday for 20 months following his guilty plea last month to one charge of sexual activity with a child by a person in a position of trust.

Judge Alistair McCreath, the Honorary Recorder of Westminster, told him it was not "rocket science" that a teacher should not have a sexual relationship with a pupil under the age of 18 at their school.

He said: "If anybody had it fresh in his mind that teachers should keep their hands off pupils, it must be a newly-qualified teacher over all others."

Stephenson, from Third Avenue, York, started seeing the girl when she was 16 and in the sixth form at Grey Coat, a girls academy where former prime minister Mr Cameron's daughter Nancy started last year and which former education secretary Michael Gove sent his daughter Beatrice to in 2014.

The court heard the pair began communicating online in the summer of 2007 after she "poked" him on Facebook.

Her father became concerned after discovering an email but Stephenson assured him the relationship was "platonic".

But the court heard it soon became sexual.

Prosecuting, Gregor McKinley said that on one occasion they went to Camden in London, got drunk and he took her back to his house and put her to bed.

He added: "On another occasion she says she was persuaded to sleep in his bed without her top on and was awoken by him taking a photograph of her without her top on.

"She hadn't consented and later saw it on his mobile phone."

Mr McKinley told the court it was not a "classic" case of grooming.

But, interrupting, Judge McCreath said: "But it is grooming, isn't it? An adult is talking to the child through electronic media in a way that is flirtatious and sexual.

"This follows lengthy electronic conversation which has at the end of it a sexual relationship. If that is not grooming, what is it?"

Their relationship continued after she turned 18 and eventually ended in 2009, with Stephenson leaving the school that year for another job.

Last year he was nominated for a teacher of the year award while head of maths at Fulford School in York, but he no longer teaches there.

Police first learned of the matter in 2011 but the girl, who cannot be named for legal reasons, only pressed charges last year.

The court heard she had been a "trusting teenager" and her life had been "turned upside down" by the incident.

Defending, Kevin Blount QC said: "This is not a man who has gone out to find someone vulnerable to take advantage of, it is a friendship that blossomed."

Stephenson - who was due to be married last month - was "young and foolish" when the relationship happened, he said, adding: "His life is in tatters at a relatively young age because of a mistake he made when he was 23 or 24."

Judge McCreath told him: "It matters because teachers should not be encouraging this sort of relationship, they should be actively discouraging it."

Jailing Stephenson - who stood impassive in the dock as he was jailed - the judge told him: "I'm afraid I can't suspend the sentence given the nature of this activity and the repetition of it."

He was also ordered to sign the sex offenders' register for 10 years.