The head of a leading women’s rights movement has called for an independent investigation into what she calls pervasive bullying and sexism within a UN agency, alleging that she was harassed and sexually assaulted while working for the organisation.

Malayah Harper, now general secretary of the World YWCA, one of the world’s oldest women’s rights organisations, said the agency UNAids must urgently review how sexual harassment allegations are handled.

Her call for an inquiry is supported by testimonies from six current or former UNAids employees. The women, who spoke on condition of anonymity, described a culture where senior male staff offer career progression in exchange for sexual relationships, and harass women with seeming impunity. “It’s a boys’ club environment,” said one employee. “They just see it as almost part of their due.”

Harper, who worked for UNAids for more than 10 years, alleges she was sexually harassed for years and sexually assaulted in 2013 by Luiz Loures, the agency’s deputy executive director. She did not report the incidents at the time because she believed no action would be taken. A previous complaint she had made, against a different member of staff which related to bullying and intimidation, had not been handled adequately, she said. The investigation report was not shared with her, no counselling was offered and the alleged perpetrator was moved on through a promotion, she added.

“I’m a feminist, I was the lead in the organisation on gender – and I had been bullied quite seriously, and then sexually harassed for years. The irony of it did not escape me,” said Harper. A bigger irony, she added, was that this was happening at the UN: “The UN is the custodian and standard setter for the human rights of women and girls.” She also said that UNAids’ policies for tackling sexual harassment were “based on reporting, on weakly enforced punitive measures – not on prevention or providing support to employees”.

Harper said she had decided to speak publicly because she believed that UNAids is failing many female members of staff and the people the UN serves. “A manager has a responsibility to ensure staff are in a safe place of work. If you do nothing, you are accountable. The environment becomes corrosive and pernicious,” she said.

UNAids said it has zero tolerance for sexual harassment and abuse, and that it had clear procedures to address complaints.

Two women told the Observer they had warned the agency’s executive director, Michel Sidibé, about Loures’s alleged behaviour. One former employee, who left in 2015, said: “I had an exit interview with Michel when I left and the first words out of my mouth were, ‘your deputy director is a sexual predator and everybody knows it. I’m telling you because you really have to do something about it.’”

UNAids said Sidibé always took complaints seriously and always acted in line with required procedures.

On Friday, UNAids said that Loures, who is also an assistant secretary-general of the UN, would not seek to renew his contract when it expires at the end of March. Loures has recently been investigated over separate claims that he assaulted a current staff member in a lift while on work travel. An inquiry, which was criticised by campaigners, found the allegations to be unsubstantiated. The agency said Loures’s decision to stand down was not related to sexual misconduct claims.

The announcement of Loures’s departure follows news that Justin Forsyth has stepped down as deputy executive director of Unicef, the UN’s children’s agency, after accusations of inappropriate behaviour toward female staff while working for Save the Children. He said his resignation was not related to past mistakes made while working for the charity. On Saturday, a number of charities, including Plan International and the International Committee of the Red Cross, disclosed the number of recent allegations of sexual misconduct or sexual abuse by staff members.

Harper said the departure of high-profile officials did not exonerate the UN of responsibility, and that it had allowed a culture of sexual harassment to fester for years. She says she was repeatedly invited to drinks by Loures, who once told her she was “a very naughty girl” because she had declined an offer in South Africa to meet him one night in his hotel room. In addition, following a work conference, Loures assaulted her in a lift, she said. “We left the conference and shared a lift together and he attempted to kiss me. In the process of me moving away, he bruised my lip. On exiting the lift he tried to get me into his hotel room and ripped a button off my shirt.”

A second woman, who wished to remain anonymous, also alleged she had been assaulted by Loures, after a work meeting. She did not file a complaint. “He grabbed me, pushed me against himself, stuck his tongue in my mouth and touched me all over. I was so shell shocked,” she claimed. “I can’t say he was extremely aggressive but he was constantly all over me putting his hands on me. I said I can’t, I don’t want this, let me go. He tried several times and then he let me go.”

A further two women alleged that Loures had pursued or sexually harassed them. “When I would talk to him he had this way of backing me up against the wall and putting his arm so you’re literally trapped, pulling me in for a hug, really insisting that we needed to go out for dinner,” said one woman.

Some women said they believed work had been taken away from them, or that their career progression was slow, because they did not comply or engage in sexual relationships.

They did not report what had happened because they did not trust the complaints system, or because they had also experienced sexual harassment at the hands of the senior personnel to whom they turned for support.

UNAids said staff were regularly informed of the channels available to raise a complaint, including a 24-hour hotline, and were offered protection when they did so, such as reassignment to another team. In recent years it has taken extra steps to prevent harassment at the workplace, it said, adding that it encourages current or former staff who have a complaint to report it.

UNAids has been criticised for its handling of a recent investigation into Loures. An internal investigation report said Sidibé had attempted to settle the assault allegation informally, despite the matter being under official investigation by the World Health Organisation’s investigations team, Internal Oversight Services.

UNAids later told the Observer that Sidibé had no role in the decision-making process of the case and that the investigation could not have been stopped through an informal resolution. “Any formal complaint received by UNAids is always acted upon in accordance with due process,” it added.

This month, the Labour MP Gareth Thomas wrote a letter to Penny Mordaunt urging that the UK, as chair of the UNAids governing body, should immediately establish an external investigation into abuses of power, harassment, assault and bullying.

“This is not just one individual, it’s more than that,” said one of the women who says she was harassed by Loures. “It’s about how you establish a system that creates perks for the chosen ones, which is a group of senior men.”

Loures did not respond to a request for comment.

UNAids added in a statement that, other than the investigation recently concluded that found the allegations to be unsubstantiated, it had received one other complaint of sexual harassment in 2009. Those allegations were also found to be unsubstantiated and both parties accepted the decision, it said.

“Informal channels of reporting and resolution require that the person making such an informal complaint expressly authorises that the matter be formally addressed,” the statement said.

“Individuals who are not satisfied with the outcome of a formal resolution process may appeal to the administrative tribunal of the International Labour Organization. They also have recourse to local national jurisdiction.”