Poland’s hard-coal reserves have almost doubled since last year, prompting a powerful union to warn about the consequences for the country’s own miners just months before a tightly contested parliamentary election.

“We hear that state companies import large amounts of coal, while coal extracted from Polish mines is not being collected,” Boguslaw Hutek, the head of the powerful Solidarity miners’ union, said in a statement. “State-owned companies should not act against the interest of each other.”

Coal stockpiles jumped 83% to 10.8 million tons in May from a year earlier, according to energy think tank Instrat. The government in Warsaw, which banned Russian coal imports in March 2022, prompted state-run energy producers to import heavily from countries like Colombia, Kazakhstan and South Africa to avoid shortages.

Coal has long been politically important in Poland, where 75,000 are employed as miners and millions of households depend on the fuel for heating. Aging coal plants provide about 70% of the nation’s electricity.

Poland is poised to hold its parliamentary election in October. Opinion polls have shown the two main parties short of the support needed to rule independently, putting smaller parties in the position of potential kingmakers after the election.