The head of a prestigious Oxford college has been accused of failing to disclose an allegation of sexual assault of a minor to police or safeguarding officials.

In the latest twist in a long-running dispute that began in 2018, Martyn Percy, the dean of Christ Church and head of its cathedral, strongly rejected the claims against him by the college, saying each of the cases were properly dealt with at the time.

One of the dean’s supporters described the college’s move as a “black ops campaign”.

The governing body of Christ Church said in a statement posted on its website that, following an investigation prompted by a media inquiry last month, “it is apparent that [the] allegation was disclosed to the dean, but never reported by him to the police, the local authority designated officer, Christ Church’s safeguarding officers, or the Church of England’s safeguarding officer”.

Christ Church said it had informed the police and the Church of England’s national safeguarding office.

Internal investigations had “raised serious concerns about the dean’s handling of four separate matters reported to him. All relate to allegations of sexual abuse or assault, two involving a minor.”

The college added: “There is no implication whatsoever that the dean himself has been involved in any form of sexual misconduct.”

Thames Valley police confirmed they had received a third-party report of a rape last month but, as the alleged victim had never reported the incident, “there is no line of inquiry and no current investigation”.

In a statement to the Guardian, Percy said he had “dealt correctly with three historic cases of reported sexual assault in the academic year 2016-17, and the information on these were shared with the appropriate college officers at the time.

“A fourth historic disclosure was made by an individual who had never reported the matter to the police, and only agreed to talk about the alleged assault on the condition that there was no further disclosure. Their position has not changed.”

In 2017, he had raised as a matter of concern that college officers who should have responsibility for safeguarding had not been made aware of this nor had appropriate training. The following year, he changed their job descriptions to reflect their responsibilities.

The college’s statement comes after fresh revelations in recent weeks about the acrimonious battle at Christ Church, one of Oxford’s most venerable colleges, over Percy’s position.

Supporters of the dean have claimed that a small group was determined to have him removed from his post after he attempted to modernise the way the college was run. Percy’s detractors insisted the dispute originated in the dean’s attempts to secure a pay rise.

After being accused of behaviour of an “immoral, scandalous or disgraceful nature” – the only charges which can result in a dean’s removal – Percy was suspended from his post in October 2018.

A retired high court judge, Sir Andrew Smith, who was hired by the college to conduct an internal tribunal, dismissed the complaints against Percy following an 11-day hearing behind closed doors in June. Percy was reinstated but the full judgment has never been published.

An employment tribunal is expected to hear Percy’s case in public next year.

Last month, leaked emails between members of the governing body revealed the depths to which the feud sank. One wrote: “I’m always ready to think the worst of him. Does anyone know any good poisoners?” Another referred to him as “little Hitler” and yet another wrote: “[Imagine] the Inspector Morse episode we could make when his wrinkly withered little body” was found in the Thames.

Earlier this week, the 60 members of Christ Church’s governing body were ordered to delete unread an email from the former Tory minister Jonathan Aitken, an alumnus of the college, which included an unredacted copy of the Smith judgment.

Geraldine Johnson, the senior governing body member, wrote to her colleagues: “It is extremely important … that no one opens the attachment, however tempting this may be. We should not let an external individual affect the proper governance of Christ Church in this way.”

Christ Church is believed to have spent more than £2m on legal and public relations fees since the dispute began. It is one of Oxford’s richest colleges, with funds of more than £500m.

Percy’s defence, funded from his own pocket, is thought to have cost around £425,000.

Last year, the Charity Commission instructed Christ Church, which is a charitable body, to order an independent review of its governance.

The commission told the Guardian: “We continue to engage closely with Christ Church college over a number of issues as part of an ongoing regulatory compliance case. That has included instructing the trustees to commission an independent governance review. The governing body has committed to undertaking the review and we are in discussions about it.”

One of Percy’s supporters said: “This looks like the latest desperate attempt by a small cabal of tutors to smear their dean. They failed to dislodge him when an independent judge exonerated him. Now, almost unbelievably, they are spending charitable funds to run a black ops campaign against their own colleague.”

Christ Church, founded by Henry VIII in 1546, is the alma mater of 13 British prime ministers, 10 chancellors of the exchequer and 17 archbishops. Among its former students are King Edward VII, Albert Einstein, Lewis Carroll and WH Auden.