Environment watchdog body, Natural Resources Wales, is asking farmers and land owners to be aware of requests to store refuse derived fuel illegally on their land.
The warning follows an increase in reports from landowners who have been approached in recent weeks.
Individuals acting alone, or operating under a company name, are offering land owners money to store large quantities of RDF, claiming it is a temporary measure before the waste is sent for incineration.
But NRW says landowners should be aware that the storage of RDF is tightly regulated, and an environmental permit is required in order to store the waste legally.
This is to make sure the material is managed and stored correctly to avoid pollution and fire risk.
NRW waste adviser, Geraint Richards, said: “We want to warn farmers and land owners that this underhand activity is on the rise both to protect their interests and the environment.
“While it may seem like a tempting proposition to make some extra cash, landowners could find themselves risking enforcement action, or substantial clean-up costs if the waste is abandoned by the perpetrators,” he added.
“Anyone approached to store RDF or any other waste type on their land should report it to us immediately so we can investigate further.”
Reports should be made through the NRW incident hotline on 0300 807060.