American teacher and prolific paedophile William Vahey drugged and preyed on up to 90 young teenage boys around the world

The scale of abuse carried out by a convicted paedophile on pupils at a private London school was described by its chair of governors as “the worst thing” he has ever been involved in during 40 years of education.

Sir Chris Woodhead, the former Ofsted Chief Inspector, confirmed ex-teacher William Vahey had preyed on children at the independent Southbank International School and said it made him feel physically sick when he heard what had happened.

American citizen Vahey, 64, killed himself last month, two days after investigators filed a warrant to search a computer drive containing pornographic images of at least 90 boys aged from 12 to 14 who appeared to be drugged and unconscious meaning many of the victims may still be unaware they were abused. It is believed that all of the boys in the images were his students going back to 2008 – Vahey taught history and geography to pupils at the London school from 2009 until last year.

It is not clear exactly how many children in England have been affected, although the FBI, which is working with the school to identify affected children, said it was searching for up to 90 potential victims from international schools around the world. Vahey also taught in Indonesia, Saudi Arabia, Madrid, Athens, Venezuela, Iran and Lebanon, managing to slip through the net for decades despite being jailed for child sex offences in California in 1969.

Sir Chris said that parents at the London school, which teaches children aged three to 18, were very concerned and that it “beggars belief” Vahey was able to teach for so long while harbouring the conviction.

He said: “Everyone at the school is deeply shocked by what we heard. Our two priorities now are to communicate as much information as we have as quickly as we can, and to help the police as much as we can in what is now an international police inquiry into the activities of this man.”

Sir Chris said Vahey, who mainly taught pupils aged between 11 and 14 but also took students out on trips, had an “immaculate record” and had never given any staff at the school any cause for concern. He said: “He was a very popular man, both with staff and students. He has managed to deceive his colleagues in schools all around the world for 30 years… This is the worst thing that I’ve ever been involved in in 40 years of education.”

A meeting at the school which police and other investigators will attend is due to be held next week. Special Agent Patrick Fransen from the FBI said: “I’ve never seen another case where an individual may have molested this many children over such a long period of time. I'm concerned that he may have preyed on many other students prior to 2008.”

Vahey’s abuse caught up with him when he was confronted about the images by a colleague at the American Nicaraguan School in Managua, Nicaragua, where he had most recently been teaching. He confessed that he was molested as a child and had preyed on boys all his life, plying them with sleeping pills before abusing them. The photos were catalogued with dates and locations that corresponded with overnight field trips that Vahey had taken with students since 2008, but he had led pupils on such outings for his entire career.

Vahey, who had a home in London as well as in South Carolina, was found dead in Luverne, Minnesota, on March 21.

Scotland Yard said it was helping the FBI with its inquiries. A spokesman said: “Officers from the sexual offences, exploitation and child abuse investigation team are assessing and evaluating intelligence passed to the Met by US authorities, and actively seeking any evidence whilst working with partner agencies to ensure that potential victims are supported.”