Oxfam's deputy chief executive has resigned after admitting the charity "failed to act" over concerns about two of its staff members' behaviour in the wake of the Haiti sexual misconduct scandal.

Penny Lawrence said she is "ashamed" that Oxfam workers were found to have paid for prostitutes while working as aid workers during the devastating 2010 earthquake, which killed more than 222,00 people.

It later emerged that Oxfam had been made aware of the behaviour towards of one of the men at the centre of the scandal – Roland van Hauwermeiren – during his time in Chad, before he was moved to Haiti.

According to the Times, Van Heuwmairen had also received at least four complaints about the behaviour of the second Oxfam staff member, which involved the use of prostitutes and sexual harassment, and still allowed him to keep his job.

The two were among seven Oxfam workers who were eventually dismissed or allowed to resign in 2011 after an investigation into the use of prostitutes, downloading pornography, bullying and intimidation in Haiti in the wake of the earthquake.

Lawrence has now announced she is stepping down from her role as the charity continues to face criticism for the way they handled the abuse allegations.

She said: "Over the last few days, we have become aware that concerns were raised about the behaviour of staff in Chad, as well as Haiti, that we failed to adequately act upon. It is now clear that these allegations – involving the use of prostitutes and which related to behaviour of both the country director and members of his team in Chad – were raised before he moved to Haiti.

"As programme director at the time, I am ashamed that this happened on my watch and I take full responsibility.

She added: "I am desperately sorry for the harm and distress that this has caused to Oxfam's supporters, the wider development sector and most of all the vulnerable people who trusted us.

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A woman carries water through the largest camp for earthquake refugees in Haiti Getty

"It has been such a privilege to work for such an amazing organisation that has done and needs to continue to do such good in the world."

Lawrence joined Oxfam in February 2006 as programme director and was later appointed deputy chief executive in 2016.

Mark Goldring, Oxfam's chief executive, said: "I deeply respect Penny's decision to accept personal responsibility. Like us, she is appalled at what happened and is determined to do what is best for Oxfam and the people we exist to help.

"I would like to place on record my sincere thanks for the years of dedicated service that Penny has given to Oxfam and the fight against poverty around the world."

Elsewhere, former international development secretary Prit Patel claimed the abuse allegations at Oxfam is just the "tip of the iceberg".

Writing in the Telegraph, she added: "Appallingly, we have actually seen child rape, sexual exploitation and abuse carried out under the UN flag – and not just by peacekeepers or in peace operations.

"These deplorable crimes have been committed against children and vulnerable people - the very people the UN is mandated to protect."

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The government announced it was reviewing all work with Oxfam amid revelations the charity's staff hired prostitutes in Haiti during the relief effort ANDY BUCHANAN/AFP/Getty Images