Welsh firm Potters Waste Management (PWM) has had its permit to operate the Bryn Posteg landfill in Powys susptended after storing “substantially more waste than is legally allowed”.
Natural Resources Wales (NRW) officers said it took action because of the pollution risk at the site.
NRW suspended the permit on 19 January, and PWM will not be allowed to deposit waste until it addresses the situation.
Last year, NRW served legal notices on PWM in an attempt to make the site compliant with its permit conditions, which restricts the company to up to 75,000 tonnes of waste at Bryn Posteg in a year.
The company is also obliged to properly treat the waste.
David Powell, NRW operations manager, said: “We now have evidence that Bryn Posteg landfill site contains significantly more waste than it is permitted to do so.
“The decision to stop waste being disposed of in the landfill is a last resort, but there is a risk to the surrounding environment as a result of over-tipping.
“Waste sites have a permit setting out rules which they have to follow so they don’t pose a risk to the environment and local people.
“We have given the company every opportunity to comply with the conditions in their permit but they have not done this.”
Under the terms of their environmental permit Potters Waste Management can tip up to 75,000 tonnes of waste at Bryn Posteg every year.
NRW’s investigations are continuing.
PWM is billed as one of Wales’ largest waste management companies.
It runs two landfill sites, at two landfill site operations at Bryn Posteg, Llanidloes and at Withyhedge in conjunction with Pembrokeshire County Council.