Ofgem has confirmed it is investigating three cases of suspected fraud related to the Renewable Heat Incentive (RHI) scheme in Northern Ireland.
The values range from £48,000 to £350,000 while the third could potentially be worth around £2.5 million.
The information was provided in response to a freedom of information request.
Under the RHI scheme, which was launched in 2014, householders and businesses receive cash payments for installing renewable heating technologies.
The regulator said the cases in question involve installations that “appear to operate in ways that were ineligible under the applicable regulations”.
It comes following allegations the scheme is flawed, with claims that users could legitimately earn more cash the more fuel they burned – and has been dubbed the “cash-for-ash” scandal.
It added: “Ofgem has a zero tolerance approach to fraud. We have a dedicated Counter Fraud team, who works to prevent, deter and detect fraud across the environmental or social programmes we administer on behalf of the government.”
Following the investigation, a decision will be made on whether to refer them to the police.
The regulator also said it had rejected 20 applications.
New statistics from BEIS revealed the RHI scheme has been adopted by less than 20,000 businesses.