A serial paedophile was able to abuse dozens of children at an elite school in London because of a failure to flag him up, or act on complaints made against him, a report has found.

William Vahey, 64, is alleged to have abused children at schools around the world for 40 years before killing himself in March after indecent images of children were discovered on his computer by a maid at a school in Nicaragua.

Vahey, who the FBI fear may have had hundreds of victims around the world, worked at the £25,000 a year Southbank International school between 2009 and 2013.

According to an independent review by barrister Hugh Davies QC, Vahey was employed by the prestigious school after it failed to carry out proper checks and then ignored complaints about him.

Vahey is estimated to have abused around 60 boys at Southbank by drugging them on school trips.

According to the report, seen by the Guardian, questions about Vahey's conduct were raised at the school during his first year of employment. A school source also said fears were raised about Vahey's behaviour on school trips.

"Complaints were made about him wanting to spend time alone with boys," the source said. "He would say he would stay behind with the boys and the lead teacher would say that was against guidelines. [The teachers] made complaints to the management."

The school's recruitment process also comes into question in the report. Apparently it did not meet statutory guidelines; there was a failure to seek references for Vahey before his interview, which was conducted by only one person, the then headmaster, Terry Hedger.

The school's background checks came into question as news of Vahey's offending emerged. In 1969, Vahey was sentenced to 90 days in jail after been convicted molesting children at a high school in California, where he taught swimming.

Despite this, he was able to take a teaching job at a school in Iran in 1972 as he failed to sign the sex offender's register, before going on to similar roles in schools in countries such as Venezuela, Spain and Greece.

Sir Chris Woodhead, the school's chairman of governors, said the school examined Vahey's employment history for the previous 17 years; Vahey had registered as a teacher in 1986 and it was assumed that to achieve this he must have had no prior convictions.

Damningly, the report states that suspicions about Vahey's "unusual/ questionable conduct" were passed on to authorities, but this was not sufficiently recorded, so the alarm was not raised.

The report claims the school's staff and child protection officers were not sufficiently trained in how to deal with complaints which led to an "under-reporting of concerns relating to Vahey".

Davies said: "Had what was reported within the school about Vahey's conduct been appropriately recorded and evaluated, the pattern of conduct recorded would have required direct intervention by the school.

"As a minimum, reviewing his role on school trips and almost certainly further independent investigation under the direction and control of the local authority designated officer."

Woodhead accepted a better system of recording should have been put in place, but said it would not have stopped Vahet from abusing the boys.

"There would have been some intervention but what that would have been is speculation," he said.

The FBI launched a worldwide appeal asking for help to identify any potential victims in April after images of at least 90 children aged between 12- 14 being abused were found on his computer.

William Vahey
William Vahey taught at school across the world for 40 years FBI