The mother of the teenage girl who fled to France with her teacher has said she felt her daughter was “dead” to her as a result of the illicit relationship.

She spoke out as Jeremy Forrest was jailed for five-and-a-half years yesterday after admitting having sex with a child. Forrest, 30, was described by a judge at Lewes Crown Court as motivated by self-interest and told he had hurt and damaged everyone around him in his determination to abuse the 15-year-old and evade justice.

The victim, who cannot be named for legal reasons, wept when her former teacher was found guilty on Thursday, telling him: “I’m sorry.”

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Forrest had intended to argue that, as the age of consent in France is 15, he had committed no crime.

Yesterday, following complex legal proceedings in both France and at the European Court of Justice in Luxembourg, he waived his rights under the terms of his extradition and admitted five counts of sexual activity with a child, boosting his sentence by four-and-a-half years. He had earlier been found guilty of abduction at the end of an eight-day trial.

Sitting in the public gallery without her daughter, who has moved out of the family home, the girl’s mother said the family had been ripped apart.

She said her daughter was angry and blamed her for what had happened, creating a break down of trust.

“I feel the [daughter] I knew is dead and it upsets me beyond words,” she said. “I feel completely useless most of the time. I feel like I have failed as a parent as I cannot understand how someone could do this to my child and I had no idea,” she said in a witness impact statement.

Judge Michael Lawson QC told Forrest it had been his duty as a teacher to stop the girl’s infatuation with him. Instead he had deliberately encouraged it: “Your behaviour in this period has been motivated by self-interest and has hurt and damaged many people – her family, your family, staff and pupils at the school and respect for teachers everywhere,” he said.

Judge Lawson put him on the sex offenders register and imposed a Sexual Offences Prevention Order banning Forrest, a former teacher at Bishop Bell Church of England School in Eastbourne, East Sussex, from future unsupervised contact with children. He will begin his sentence at Lewes Prison. As a sex offender he will spend most of his time in jail in a vulnerable prisoner unit.

The NSPCC said warning signals had been ignored by the school’s staff.

David Tucker, associate head of policy at the charity, said: “Some of his colleagues knew about the unhealthy relationship but did not intervene to prevent it escalating to abduction.”

A review by the Local Safeguarding Children Board will now examine the lessons learned from the case.

Forrest groomed the girl for several months and started having sex with her shortly after her 15th birthday. The couple exchanged indecent images and were seen holding hands on a school trip. When police began to make inquiries seven months after initial concerns were raised, Forrest abducted the child, escaping to France and sparking a major man hunt. He spent seven days on the run with the girl but was eventually spotted after trying to get a job in a bar in Bordeaux.

Forrest’s family said he was “very sorry” for his actions. In a statement they said: “Despite the verdict and today’s sentence, there are many factors in this case which need to be examined and addressed, including the failure to properly act on early warnings.”