The scandal surrounding Oxfam has deepened after a whistleblower claimed the charity received three allegations of child abuse in its shops.
Helen Evans, who was head of safeguarding at Oxfam from 2012 to 2015, said she raised fears that the charity was failing to report incidents and that the number of cases "was likely to be far wider than was being reported".
In a lengthy statement on Twitter, Ms Evans claimed the allegations surrounding Oxfam's work in Haiti were not an "isolated incident" and claims began to "flood in" as reporting mechanisms were strengthened.
In response to Ms Evans' claims, an Oxfam spokesperson said: "We regret that we did not act on Helen's concerns much quicker and with more resources."
They said the charity had "introduced a whole range of measures to improve how we deal with safeguarding issues".
"We ensure all staff are trained in working with young people and vulnerable adults, have appropriate background checks and know how to respond to any issues raised," the spokesperson added.
Ms Evans also hit out at the Charity Commission, the Government and the Children's Commissioner over the claims, alleging she raised her concerns with all of them in 2015 but no action was taken. Sky News has contacted them for comment.
The whistleblower's claims come after it was revealed an inquiry has been launched into Oxfam by the Charity Commission.
The investigation will examine concerns it did not disclose all of the details about its 2011 investigation into the claims.
Oxfam has been accused of covering up the use of prostitutes by staff in Haiti in the wake of the deadly earthquake in 2010. Allegations that prostitutes were used by staff in Chad in 2006 have also emerged.
It has denied a cover-up, but the charity's chief executive has said nine members of staff "behaved in a way that was totally unacceptable".
The organisation's deputy chief executive quit earlier on Monday, saying she was "ashamed" of what happened.
Penny Lawrence said she took "full responsibility" for the behaviour of the charity's staff in Chad and Haiti "that we failed to adequately act upon".
In a statement released after a meeting with senior Oxfam figures, International Development Secretary Penny Mordaunt said: "In the 21st century, it is utterly despicable that sexual exploitation and abuse continues to exist in the aid sector.
"I am determined that we do our utmost to prevent exploitation and abuse happening - and ensure that where it does happen it is identified and dealt with appropriately."
She revealed that Oxfam "made a full and unqualified apology" to her, the British public and the people of Haiti for the "appalling behaviour".
"They spoke of the deep sense of disgrace and shame that they and their organisation feel about what has happened, and set out the actions they will now take to put things right and prevent such horrific abuses happening in future," Ms Mordaunt said.
Oxfam chair of trustees Caroline Thomson, who was at the meeting, said the charity "unreservedly apologised to her (Ms Mordaunt) as well as to our supporters, donors and the people of Haiti for the things that happened in our name".
The charity has been set a deadline of the end of the week to detail how it will handle any further allegations "in a way in which the public can have confidence", Ms Mordaunt revealed.
The scandal has put Oxfam's funding from the Government under threat, with the charity receiving £31.7m in 2016/17.
It has also raised questions as to whether such exploitation and abuse is more widespread in the aid sector.
Ms Mordaunt's predecessor, Priti Patel, wrote in a newspaper article on Monday that the Oxfam claims were "the tip of the iceberg".
And while seeking to keep up the pressure on Oxfam to act, Ms Mordaunt's latest intervention was designed to throw the spotlight on the sector as a whole.
She said charities need to "step up and do more" in the wake of the Oxfam allegations.
As part of a series of measures, the International Development Secretary has:
:: Written to all UK charities operating overseas, demanding they "step up and do more" so "we have absolute assurance that the moral leadership, the systems, the culture and the transparency that are needed to fully protect vulnerable people are in place"
:: Asked such charities to confirm they have referred any cases or concerns about individuals to the relevant authorities
:: Established a new unit to "urgently review" safeguarding across the sector to ensure people are being protected from sexual exploitation and abuse
:: Pledged to step up work to tackle the problem at the UN and other international bodies
:: Announced the department will co-host a summit on the issue by the end of the month along with the Charity Commission, and involving charities that operate overseas