The head of Boris Johnson's former prep school has been arrested and questioned on suspicion of historical sexual abuse.

Clive Williams, 69, was interviewed by Sussex police on Wednesday following allegations of sexual assault and child neglect. A computer and documents were taken from his home for examination.

The former head of Ashdown House prep school in Forest Row, East Sussex, was released on bail the same day and is understood not to have been charged. Williams was head at the school, of which he was also a former pupil, from 1975 to 2003.

Founded in 1843, the 125-pupil school has been owned by the Cothill Educational Trust since 2009. Former pupils include the mayor of London, his sister, Rachel, and the actor Damian Lewis.

A police statement said Williams was released on police bail until 11 November while inquiries continued.

"During January this year we were contacted by a firm of solicitors representing a number of clients who have reported to them that they were sexually and physically assaulted by staff while they were pupils at Ashdown House school near Forest Row in the 1970s," the statement said.

Police said they had contacted 20 people reporting offences, including clients of that firm. "Most have already been interviewed by officers from our specialist child protection team in East Sussex. Arrangements are in hand to interview the others, some of whom live abroad. The investigation continues and now spans a period from the late 1960s to the early 1980s."

The statement added: "Reports of this kind are always taken seriously, however long ago the offending is said to have occurred."

Police emphasised there were no current safeguarding issues at the school relating to the inquiry. "At this stage we are not disclosing the number of people against whom allegations have been made."

The trust said in January it had been "given to understand that it may be the subject of legal proceedings relating to episodes of abuse said to have taken place between 30 and 40 years ago".

It added that trustees were "deeply saddened" and wished to do everything they could to assist any former pupil who had been affected. The school received "informal approaches" from two former pupils in 2003 and advised them that the matter should be reported to the police, it said.

Sussex police encouraged anybody with relevant information, including any former pupils who may have been abused at the school during the period and had yet to come forward, to make contact by phoning 101 or emailing 101@sussex.pnn.police.uk, quoting Operation Mitre.