The former headmaster of a prep school attended by London mayor Boris Johnson has been arrested on suspicion of historic sex assaults.
The suspect, confirmed by sources as 69-year-old Clive Williams, was also questioned by officers on Wednesday over allegations of child neglect.
Mr Williams was headmaster at Ashdown House Preparatory School in Forest Row, for more than 25 years before leaving in 2003.
A computer and documents were seized by police investigating the claims against Mr Williams before he was freed on police bail on the same day.
He has not been charged.
A Sussex Police spokesman said: "Officers investigating allegations of historic sexual abuse at a school in East Sussex have arrested a local man.
"The 69-year old was arrested at an address in Barcombe on Wednesday July 9 on suspicion of sexual assaults and child neglect.
"A computer and documentation were seized for examination. After being interviewed, the man was released on police bail on the same day until November 11, while inquiries continue."
Sussex Police have contacted 20 people who have reported sexual and physical offences against them by school staff, and most have been interviewed by officers from the child protection team.
The police spokesman added: "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."
Founded in 1843, the 125-pupil Ashdown House School has been owned by the Cothill Educational Trust since 2009.
In January, the Cothill Educational Trust issued a statement saying it might face legal proceedings over allegations of abuse dating back up to 40 years.
It said the "trustees are deeply saddened about those matters and wish to do everything they can to assist any former pupil who has been affected".
Sussex Police added: "We wish to emphasise that there are no current safeguarding issues at the school in relation to this.
"At this stage we are not disclosing the number of people against whom allegations have been made, while the inquiry continues."
The force urged anyone with information relevant to their inquiry, including ex-pupils who have been abused during the period to call them on 101, quoting Operation Mitre.