The US Environmental Protection Agency said it cannot make large oil refineries blend additional biofuels into their gasoline to make up for volumes waived under the small refinery hardship waiver programme unless Congress authorises it.

The agency’s decision was certain to anger the powerful US corn lobby and Congressional members from farming states.

EPA has issued a record number of waivers freeing small refineries from the US Renewable Fuel Standard (RFS) in the past year, drawing criticism from the biofuel industry and its legislative backers that the program undermines demand for corn-based ethanol.

Lawmakers like Republican Senator Chuck Grassley of Iowa have demanded that other refiners take on bigger obligations to compensate for the volumes exempted under the program, as a way for President Donald Trump’s administration to keep its promise to protect the 15-billion-gallon biofuel mandate.

The US Renewable Fuel Standard requires refiners to blend specific volumes of biofuels like corn-based ethanol into the fuel supply each year and prove compliance by acquiring credits called RINs that can be either earned or purchased. The law has helped farmers by creating a big market for ethanol, but refining companies have complained it costs them a fortune.