Female whistleblowers and global HIV activists have urged the British government to demand the resignation of a senior UN official over his alleged mishandling of serious sexual harassment claims.

The high-profile group of campaigners, which includes Martina Brostrom, who maintains she was sexually assaulted by a former UN assistant secretary general while she worked for UNAids, wants Theresa May and the Department for International Development to intervene over the conduct of the agency’s executive director, Michel Sidibé.

The UK will chair the organisation’s programming board when it meets in Geneva in June.

At a press conference in London, speakers also called on Britain to use its influence to oversee root-and-branch reform of the UN organisation, which they depicted as a “boy’s club” where sexual harassers were able to act with impunity.

Thursday’s press conference was the latest intervention in the controversy that has been swirling around senior management at UNAids, not least Sidibé, who has been accused of mishandling a sexual harassment investigation.

In February, the Guardian reported concerns relating to the conduct of an investigation into Brostrom’s allegation that she had been assaulted by Sidibé’s deputy, Luiz Loures. The allegations were denied by Loures, who was cleared of wrongdoing by the investigation.

Among other allegations, Brostrom has claimed that Sidibé attempted to bribe her to drop her complaint by offering a promotion. Sidibé has denied this, but admitted to investigators that he had met Brostrom to suggest they “see how we can really find a way out without making it a big problem for all the organisation and for the credibility of the organisation, for yourself”.

Under fire: UNAids head Michel Sidibé. Photograph: Rick Bajornas/UN

Sidibé faced further criticism after details of an all-staff meeting were leaked to the Guardian, during which he apparently suggested that women who have spoken out “don’t have ethics”, and seemingly issued veiled threats to whistleblowers in his organisation. Sidibé told staff present at the meeting that suggestions he had been warned about Loures were “total lying”.

Addressing an audience that included activists wearing T-shirts printed with the words “Step Down Sidibé!”, Brostrom – who is currently on medical leave – complained that she had seen no evidence of accountability over her allegations, despite the opening of a new internal investigation.

Saying she had “exhausted” all the UN’s internal mechanisms “to no avail”, she added: “I am here because I want to seize the opportunity to ask Theresa May to intervene to stop my torment and the torment of other victims of sexual harassment and rape in the UN system.

“The UK is in a unique position to make sure sex violence does not go unpunished.

“Since going public in March the support I have had from around the world has been tremendous and humbling. The UN has been unresponsive, with the spokesman for the UN secretary general [António Guterres] instead reiterating the secretary general’s support for Sidibé, who has covered up the sexual assault against me while knowing that Loures’ sexual assaults were an open secret in the the organisation.”

A Department for International Development spokesperson said: “An independent expert panel will be appointed to carry out a review into UNAids. We will expect them to look at all the allegations raised and to make recommendations for what needs to change.

“We have been very clear that we will not work with any organisation that does not live up to the high standards on safeguarding and protection that we require.”

Thursday’s event has followed mounting concern over the damage that critics say is being done by Sidibé’s continued presence at the top of UNAIDS even among those who concede his contribution to the global fight against Aids.

Writing in the Lancet last month, Richard Horton, the medical journal’s editor-in-chief, said the UN should resist calls for Sidibé “to resign or to be fired”, but insisted the correct response while allegations of harassment, bullying and abuse are being investigated would be for Sidibé to “voluntarily suspend himself during the period of this inquiry”.

Zoya Shabarova, head of the Aids Healthcare Foundation’s European office, said: “We are calling on Michel Sidibé to stand down because he has lost all credibility after his mishandling of allegations of sexual harassment and assault. He has damaged [UNAids’] reputation and is morally unfit to lead the organisation while fostering a toxic culture of lack of accountability, impunity abuse and cover-ups.”