Oxfam has been issued with an official warning after an inquiry found evidence of mismanagement among staff sent to help victims of the 2010 Haiti earthquake.

There was a “culture of poor behaviour” among Oxfam staff, with serious allegations of wrongdoing including sexual abuse of children which were not fully disclosed, the Charity Commission for England and Wales said.

The lengthy report, published on Tuesday after an 18-month investigation, found the charity failed to heed warnings – including from its own staff – that its culture and response to keeping people safe was inadequate, and that subsequent commitments to improve safeguarding were not backed up by actions.

Caroline Thomson, Oxfam GB’s chair of trustees, said the findings into the organisation’s behaviour “are very uncomfortable”, adding: “What happened in Haiti was shameful.”

She said the charity was “deeply sorry” and said: “It was a terrible abuse of power, and an affront to the values that Oxfam holds dear.

“The Commission’s findings are very uncomfortable for Oxfam GB but we accept them.

“We now know that the 2011 investigation and reporting of what happened in Haiti was flawed – more should have been done to establish whether minors were involved.

“While the Commission makes clear that it found no record of a ‘cover-up,’ we accept that Oxfam GB should have been fuller and franker in its initial reporting of the allegations.”

The findings said Oxfam failed to adequately investigate allegations that children as young as 12 or 13 were victims of sexual misconduct against a charity “boss”, that it did not report allegations of child abuse by charity staff in Haiti, and that senior staff implicated in sexual misconduct claims were dealt with more leniently than junior figures.

Charity Commission chief executive Helen Stephenson said: “What went wrong in Haiti did not happen in isolation.

“Our inquiry demonstrates that, over a period of years, Oxfam’s internal culture tolerated poor behaviour, and at times lost sight of the values it stands for.”

She said “significant further cultural and systemic change” is required at Oxfam – which has been under the leadership of new chief executive Dhananjayan Sriskandarajah since January – to address the failings and weaknesses.

Oxfam was plunged into crisis in February 2018 when it emerged that some of its workers engaged in “sex parties” with prostitutes after the humanitarian disaster in the Caribbean country.

The Charity Commission launched its inquiry amid concerns Oxfam may not have fully and frankly disclosed material details about the allegations at the time in 2011, its handling of the incidents since, and the impact these have had on public trust and confidence.

The report said the incidents in Haiti identified in 2011 were not “one-offs”, with evidence of behavioural issues as early as June 2010.

Oxfam’s internal investigation, following allegations by a whistleblower in 2011, identified four staff who either did or were suspected of using prostitutes, including on charity residential premises.

The charity could not conclude whether minors were involved in some of the incidents, something the regulator said should have been looked into more closely.

Separately, two allegations of physical abuse, made by email from a 12-year-old and a 13-year-old girl, were “suspected” not to be genuine by Oxfam at the time.

The Charity Commission said that despite there not being evidence to back up the allegations, Oxfam should have tried harder to substantiate the claims at the time.

The report, which also examines Oxfam’s conduct in the years after the Haiti allegations, found Oxfam failed to make sufficient “full and frank disclosures” about allegations against the charity in 2011, but said there was no evidence of a cover-up.

The 142-page report, which involved 34 interviews and around 7,000 pieces of evidence, also investigated Oxfam’s conduct since 2012.

It found there were “systematic weaknesses” in its attitude to safeguarding, and there was no up-to-date safeguarding strategy in place as recently as 2018.

This included weaknesses in human resources practices, including on vetting, referencing, recruitment and induction.

There was also a failure to consistently hold people to account for poor behaviour and to ensure robust and consistent action was taken, resulting in a culture of tolerance of poor behaviour, the regulator’s investigation concluded.

This was likely to have resulted in putting victims off speaking up.

The report found the risk to and impact on victims “appeared to take second place at times” and was not taken seriously enough, and that victims, whistleblowers and staff who tried to raise concerns were let down.

The Charity Commission said it had exercised its legal powers and issued an official warning under the Charities Act on the grounds “there has been some areas of mismanagement in relation to Haiti and its safeguarding governance prior to 2018”.

It also instructed Oxfam trustees to submit an action plan to the regulator on how it will address concerns about its previous conduct, in an effort to “repair public trust and confidence” in the charity.

The allegations were described by Prime Minister Theresa May in 2018 as “absolutely horrific” and resulted in celebrities including actor Minnie Driver and singer Tallia Storm quitting their involvement with the charity.

Oxfam’s chief executive Mark Goldring also announced he would stand down at the end of the year in order to allow fresh leadership to navigate past recent criticism.

He rejoined Oxfam in 2013, about three years after the scandal.