Oxfam was hit with new allegations of staff involvement with prostitution on Saturday, after claims that employees at a second country mission had used sex workers while living at the organisation’s premises.

Former staff who worked for the charity in Chad alleged that women believed to be prostitutes were repeatedly invited to the Oxfam team house there, with one adding that a senior member of staff had been fired for his behaviour in 2006.

Roland van Hauwermeiren, who has since been embroiled in a sexual misconduct scandal in Haiti , was head of Oxfam in Chad at the time. Van Hauwermeiren resigned from Oxfam in 2011, after admitting that prostitutes had visited his villa in Haiti. One former Chad aid worker said on Saturday: “They would invite the women for parties. We knew they weren’t just friends but something else.

“I have so much respect for Oxfam. They do great work, but this is a sector-wide problem,” the former staffer told the Observer .

There are fears among aid workers that such disclosures might reduce donations to the most vulnerable at a time when funding is already under pressure.

“Charities need to find a way to deal with it ? so that people don’t reappear somewhere else,” they said.

Oxfam said it could not confirm whether it had any records about a Chad staff member dismissed in 2006. Its staff in Chad at the time lived under a strict curfew due to security concerns: employees could not walk around freely and were confined to the guest house from early evening. Some employees had raised the issue of prostitutes with Van Hauwermeiren.

Oxfam said on Saturday: “After the investigation in 2011 we carried out a thorough review of the case, which resulted in the creation of our dedicated safeguarding team, a confidential ‘whistleblowing’ hotline and safeguarding contact point within countries as part of a package of measures to ensure that we do all we can to protect our staff, prevent sexual abuse and misconduct happening in the first place, and improve how we handle any allegations.

“Our code of conduct now stipulates: ‘I will also not exchange money, offers of employment, employment, goods or services for sex or sexual favours.’ In 2011 the code only prohibited sex with beneficiaries and anyone under 18.”

Oxfam’s beleaguered chief executive, Mark Goldring, denied suggestions the charity had covered up revelations that staff had hired prostitutes in Haiti during a 2011 relief effort on the earthquake-hit island. His defence of Oxfam’s handling of the scandal came as Britain’s charity regulator said Oxfam had failed to mention allegations of abuse of aid beneficiaries in Haiti and potential sexual crimes involving minors in a report to it in 2011. It took no further action at the time.

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The scandal broke on Friday when the Times revealed that senior Oxfam staff had paid earthquake survivors for sex and that a confidential Oxfam report had referred to a “culture of impunity” among aid workers in Haiti. The Times on Saturday said Oxfam did not tell other aid agencies about the behaviour of staff involved after they had left to work elsewhere.

Goldring told BBC Radio 4’s Today programme on Saturday: “With hindsight, I would much prefer that we had talked about sexual misconduct, but I don’t think it was in anyone’s best interest to be describing the details of the behaviour in a way that was actually going to draw extreme attention to it.”

He said any references given to staff involved in the scandal had not been officially sanctioned.

The charity had been “very open” with the public that it was ashamed of the behaviour of staff and still was, he said.

Oxfam added in a statement: “With up to 10,000 NGOs working in Haiti alone in 2011, not to mention hundreds of thousands of aid workers in countries around the world, it was unfortunately not possible for Oxfam to ensure that those found guilty of sexual misconduct were not re-employed in the sector.”

Additional pressure was brought to bear by the prime minister’s office, which urged the charities watchdog to conduct a “full and urgent investigation” into Oxfam. A spokesperson for Theresa May said: “We want to see Oxfam provide all the evidence they hold of the events to the Charity Commission for a full and urgent investigation of these very serious allegations.”

Megan Nobert, a human rights lawyer, who founded Report the Abuse, the first organisation to challenge the silence surrounding sexual violence in the aid sector, said that the incident in Haiti was “a catalyst for change within Oxfam, and they are now leading the way to address this issue within the humanitarian community”.

Penny Mordaunt, the international development secretary, said in a statement: “My absolute priority is to keep the world’s poorest and most vulnerable people safe from harm. In the 21st century, it is utterly despicable that sexual exploitation and abuse continues to exist in the aid sector. The horrific behaviour by some members of Oxfam staff in Haiti in 2011 is an example of a wider issue on which DfID is already taking action, both at home and with the international community via the UN.

“We will do everything in our power to support the vital work of the Charity Commission to properly regulate UK charities that work overseas. This week I will meet with the Charity Commission to discuss what more we can do. And I am writing to all UK charities which receive UK aid, insisting that they spell out the steps they are taking to ensure their safeguarding policies are fully in place and work properly, declare all safeguarding concerns they are aware of, and confirm they have referred all concerns they have about specific cases and individuals to the relevant authorities.

“With regard to Oxfam and any other organisation that has safeguarding issues, we expect them to cooperate fully with such authorities, and we will cease to fund any organisation that does not.

“At the global End Violence Solutions summit in Stockholm next week, I will firmly demand all donors and development organisations show leadership and take action alongside the UK on this issue. I am very clear: we will not work with any organisation that does not live up to the high standards on safeguarding and protection that we require.”