A vote allowing a sex abuser to remain a fellow at a prestigious educational society has provoked a fierce internal backlash and demands for the organisation’s reform.

Scores of fellows at the Society of Antiquaries of London, a charity that promotes the study of the past, are up in arms about the vote, which has allowed Hubert Chesshyre to continue as a fellow.

The Independent Inquiry into Child Sexual Abuse (IICSA) heard evidence that Chesshyre, an expert on heraldry and genealogy who held a number of senior positions within the royal household, was found to have sexually abused a teenage chorister over the course of three years in the 1990s.

The 2015 finding was made following a “trial of the facts”, which is held when someone is considered unable to plead due to their poor mental and physical health. As a result, despite being found to have committed the abuse, Chesshyre – who is said to have dementia – was given an absolute discharge.

The finding saw fellows at the society, one of Britain’s oldest educational institutions, granted a royal charter in 1751, table a resolution demanding his removal. But a majority of fellows who voted backed Chesshyre, a former president of its elite dining club, the Cocked Hat Club.

In an open letter published on Sunday in the Observer, many fellows expressed outrage at the decision.

Pledging their support to the victim, they signalled their “determination to reform the organisation so that it reflects the values and behaviours that should be expected from any public organisation or individual”.

They add: “The voting arrangements were such that only around 100 of the society’s 3,000 fellows were able to attend the vote, which due to the existing governance structures of the society only allowed voting in person on a weekday afternoon. This disenfranchised a large number of fellows unable to attend at such a time.

“The 76 fellows who voted against the proposal to remove Mr Chesshyre’s fellowship do not represent us, and do not represent the values and behaviours of any organisation we are willing to be members of.”

Paul Drury, the society’s president, denied that the vote showed it had “stood by” Chesshyre. “We are committed to acting in ways which are consistent with our status as an educational charity operating for the public benefit, and as an institution which confers public recognition of the achievements of its fellows,” he said in a statement on its website.

“We are therefore actively working on the reform of our statutes, to enable swift action to remove fellowship from those who do not live up to the society’s expectations of integrity and good character.”

Drury added: “The society unreservedly apologises to the victim for any hurt the defeat may have caused.”