A global women’s health organisation has launched an independent investigation into claims that it operated a “paralysing” culture of racism and bullying.

The International Women’s Health Coalition (IWHC), which advocates for women and adolescent girls, will also conduct an internal review.

It’s the second global women’s group to recently announce investigations into racism allegations. In June New York-based international advocacy group Women Deliver launched an independent investigation into allegations of discrimination there. Its CEO, Katja Iversen, has taken a leave of absence until the investigation concludes.

The IWHC move follows a series of allegations from former staff, who say their experiences caused “racial trauma we continue to grapple with long after leaving the organisation”.

In an open letter published last week, a group of black women and women of colour called for the resignations of three senior staff members, including current president, Françoise Girard.

Addressed to Kathleen Regan, chair of IWHC’s board, the letter described a “toxic and racist organisational culture with almost all the power held in the hands of white women at the top of the organisation”. It accused some managers of publicly humiliating, intimidating and belittling women, which they say has been “an open secret for far too long”.

“We, as former black women and women of colour staff at IWHC, have collectively chosen to break our silence to publicly name the institutional racism and bullying we faced, so that it can no longer be wilfully ignored,” read the letter.

“We have consistently found ourselves facing an institutional culture that inherently dismisses and discriminates against us.”

Demanding a public apology from the organisation “for participating in and upholding racist behaviours and wilfully ignoring reports of our experiences”. The signatories called for resignations, higher staff salaries and for former staff to be released from non-disclosure agreements.

Some of those behind the letter told the Guardian they wanted to remain anonymous because of their traumatic experiences.

“We were humiliated in public meetings, and when we managed to speak up to share an idea or feedback, it was often ignored or repeated by somebody else, usually a white woman, who then got the credit. We were routinely gaslit in private meetings when we tried to reach out for help.

“After years of this, you have to dig real deep to keep showing up each day in order to pay rent.”

The New York-based IWHC lobbies on sexual and reproductive health and rights internationally and provides grants to feminist groups. Its 2019 annual report shows it distributed more than $2.8m (£2.2m) in grants to more than 100 organisations and activists around the world while 65% of IWHC’s revenue came from foundations, including the Open Society Foundation, the Oak Foundation and the David and Lucile Packard Foundation.

The IWHC board said in a statement that it “takes discrimination, racism, and bullying seriously, and apologises to anyone impacted by such behaviour”.

Regan told the Guardian on Thursday she could not comment on specific allegations, but confirmed an investigation was underway.

“We believe very strongly as a social justice organisation that we need to hold ourselves to a higher standard and we intend on doing that,” she said. “We are committed to coming out of this stronger. The organisation has critical work to do, we have a critical role to play.”