Save the Children has apologised to female employees after the charity's former chief executive admitted he had made "unsuitable and thoughtless" comments.
Justin Forsyth was twice investigated after concerns were raised about his conduct in 2011 and again in 2015, involving three women in total.
Mr Forsyth, who is now deputy executive director of Unicef in New York, admitted making "some personal mistakes" and said he had “apologised unreservedly” to the workers at the time.
On each occasion - following an inquiry carried out in conjunction with an independent trustee - the matter was considered to be closed.
However, the charity said that concerns were subsequently raised with the trustees that "matters should not have been left as they were" and a further review was required.
That review found that the human resources process "had not been followed in every aspect", the charity said.
It comes after Brendan Cox, the widower of murdered MP Jo Cox, admitted that he also made "mistakes" and behaved in a way that caused some women "hurt and offence" while working at Save the Children.
Mr Cox and Mr Forsyth had previously worked together at 10 Downing Street under Gordon Brown.
Mr Forsyth said in a statement: "I made some personal mistakes during my time at Save the Children.
"I recognise that on a few occasions I had unsuitable and thoughtless conversations with colleagues, which I now know caused offence and hurt.
"When this was brought to my attention on two separate occasions, I apologised unreservedly to the three colleagues involved.
"My apologies were accepted and I had thought the issue closed many years ago. The concerns were handled through a process of mediation and no formal complaint was made.
"I also want to make clear I played no role whatsoever in the disciplinary proceedings against Mr Cox nor their outcome.
"I was rightly recused from the case from the outset. This was handled professionally by an independent legal firm with the board and HR."
It comes as Save the Children and Oxfam have been quizzed separately by MPs regarding sexual misconduct by workers.
Unicef said it was aware of the past complaints against to Mr Forsyth.
A spokeswoman said: "We welcome Mr Forsyth's decision to come forward and acknowledge past mistakes.
"We are discussing this matter with Mr Forsyth and his former employer so we can take appropriate action."
On Sunday, Save the Children's current chief executive Kevin Watkins announced a "root and branch review" of the charity's "organisational culture", including measures to preserve staff safety and "any behavioural challenges among senior leadership".
A Save the Children spokesman said: "We apologise for any pain these matters have caused and sincerely hope that the complainants feel able to help us with the review in the coming weeks.
"This is so that we can better support our skilled and highly valued staff as they help change the lives of millions of children around the world every day."