A paedophile is likely to die in jail after being handed 13 life sentences for a "horrific" catalogue of abuse on children in Britain and abroad amid fears the full scale of his crimes have yet to emerge.
Retired English teacher Mark Frost, formerly known as Andrew Tracey, 70, pleaded guilty at the Old Bailey to 45 sex offences against nine children in Thailand between 2009 and 2012.
He also admitted having sex with two pupils in Worcestershire in England over three years in the 1990s.
On some occasions, the abuse happened on school grounds and on others, unmarried Frost's adopted son was present, the court heard.
He was sentenced by Judge Mark Lucraft to serve a minimum of 16 years for each life sentence.
Other determinate sentences were ordered to run concurrently.
In addition to the 13 life sentences, Frost was handed 10-year terms for other sex offences and five-year terms for indecent images. He made no reaction as he was sentenced.
The judge told Frost he was responsible for "the most appalling catalogue of sexual abuse" and it was clear he had an "ongoing obsession" with young boys.
He said: "Your conduct towards each and every one of these victims is horrific and deeply disturbing."
The court heard how Frost raped impoverished Asian boys and encouraged them to engage in sex acts after he groomed them with cash, sweets, computer games and swims in his pool.
He used violence to "control" his victims, hitting them around the head and with a belt, the court heard.
On one occasion, he dragged a boy out of a car with a belt around his neck and threatened to throw him down a well.
He skipped bail to avoid prosecution in Thailand but was extradited from Spain last year after his activities were uncovered by Dutch police on the computer of a man in the Netherlands.
Frost got the boys aged between 10 and 14 to give thumbs-up signs and make love heart gestures with their hands while being filmed on a webcam engaging in sexual acts.
The Dutchman they knew as Simon watched the abuse online and made suggestions for what they should do to each other, the court heard.
Frost admitted a raft of charges including multiple rapes, sex assaults, inciting children to engage in sexual activity and making indecent pictures.
He also pleaded guilty to abusing two former pupils of a school in Worcestershire in the 1990s. One of them has since died.
Frost had sex with his late victim in a school store room, during breaks, and at his home where he lived with his adopted son.
In a statement read in court, the victim said his life had been blighted by what happened and he lived in "constant fear" that the video Frost made would come out.
The other vulnerable victim said he had sex at Frost's home, in a barn and during a camping holiday in Wales and was left "scared and embarrassed".
The now-grown man told police he was rewarded with cigarettes, money and "nice things", but had decided to come forward to stop Frost abusing anyone else.
The National Crime Agency believes Frost may have assaulted many more youngsters he had contact with through his 25-year career in schools in east London, Hertfordshire and Worcestershire and as a senior Scouts volunteer.
He was brought to justice through a complex international investigation involving the NCA and authorities in Thailand, the Netherlands and Spain.
In 1978, he subscribed to the Paedophile Information Exchange (PIE), an international organisation of people who traded obscene material.
In 1986, the unmarried teacher tried to adopt a son through the Catholic Church and was turned down, only to succeed through a local authority, the NCA said. The pair are now estranged.
In 1992, he was fined for possessing indecent photographs of a child and the following year was jailed for a year for allowing his premises to be used for drugs and sex with an under-age girl.
Three years later he was banned from teaching by the Department for Education.
In 1998, he was jailed for a year for indecent assault on a male under 16 and put on the sex offenders register for 10 years.
Once he was taken off the register, he moved to Thailand.
The Asian boys' mothers expressed their "sorrow" at finding out the man they regarded as a "kind and generous foreigner" had taken advantage of their children.
Ruona Iguyovwe, from the Crown Prosecution Service, said: "The harrowing evidence presented by the prosecution in this case outlined the suffering that Mark Frost caused to his victims.
"Over many years, Frost repeatedly exploited vulnerable young victims, both in the UK and in Thailand, for his own sexual gratification.
"His offending has caused severe psychological harm to all of the children he abused, many of whom are now old enough to realise the enormity of what happened to them."