Ministers have widened their inquiry into sex abuse by aid workers to include hundreds of private firms and contractors employed by the government in disaster zones and developing countries, amid concern that the scandal may stretch beyond the charity sector.

The Department for International Development (DfID) has written to all its contracted private-sector firms, as well as UK and non-UK-based charities, insisting they immediately report any abuse allegations.

Last night two big DfID contractors said they were aware of allegations of sexual misconduct or harassment involving their staff over the past three years. A DfID source said the widening of the scope of inquiries reflected the determination of Penny Mordaunt, secretary of state for international development, to “ensure everything is being done to protect people from harm, including sexual exploitation and abuse”.

The aid sector is already reeling from allegations that charities including Oxfam, Save the Children and Unicef mishandled claims of sexual misconduct. Oxfam’s work in Haiti was temporarily suspended pending an investigation into how it handled claims of former staff paying for sex.

But campaigners have warned that private companies, which are increasingly used to deliver aid and other programmes, should be under equal scrutiny. The amount of DfID aid money paid to contractors has surged from £540m in 2010-11 to £1.34bn in 2015-16. Last year, a report by the international development select committee warned that DfID relied too much on such companies regulating themselves.

In her letter to charities and contractors, Mordaunt demands that they “specifically confirm that you have referred any and all concerns your organisation may have on specific cases and individuals to the relevant authorities”.

Dyan Mazurana, associate research professor at Tufts University in the US, who has studied the way aid charities tackle sexual abuse between staff members, said there were concerns about some employees of private contractors: “Given that we know there’s a relationship between security providers and people who are harassing and assaulting aid workers, and we know that a lot of the security providers are private contractors, clearly there’s an issue.”

Megan Nobert, a human rights lawyer and founder of Report the Abuse, the first organisation to challenge the silence around sexual violence in the aid sector, said: “Vendors and contractors should be held to similar standards as humanitarian organisations, particularly if they are working in or around humanitarian action.”

Development Alternatives Incorporated (DAI), a company used by DfID in countries including Nigeria, Ethiopia and Pakistan, said its Europe branch had investigated and reported to DfID a case involving two employees in the Democratic Republic of the Congo. The allegations were considered unfounded.

Another, Palladium, said it had received allegations of sexual misconduct or sexual harassment and had reported them all to the government or donors involved.

Adam Smith International (ASI), which has been embroiled in controversy over its police programmes in Syria, said it was conducting an internal review into historic cases of sexual misconduct. An aid project training a civilian police force in Syria, run by ASI, was suspended last year amid allegations that money was given to jihadists. ASI strongly denied the allegations and the scheme was recently resurrected following a foreign office investigation.

Stephen Twigg MP, chair of the international development select committee, said the public had a right to expect the highest standards “whether it comes from a charity, or an individual carrying out work on behalf of a contractor employed by the DfID”.

The consulting firm PricewaterhouseCoopers, which led a recent £300m DfID project aimed at improving education for girls, did not comment on the number of cases it has reported.

A spokesman said: “DfID has already introduced tough sanctions for human rights abuses including sexual exploitation in all new contracts, and requires all suppliers to have a whistleblowing hotline. Contracts are legally binding and a breach of contract can result in their termination.”