The massive Drax complex in northeastern England was opened in the 1970s based on coal, belching out huge amounts of the greenhouse gases blamed for global warming.
The European Commission, the EU's executive arm, said it "fully supports member state efforts to increase the use of renewable energy and pursue EU energy and climate objectives."
"At the same time, EU state aid rules make sure that the cost of such support for consumers is limited and does not give certain operators an unfair advantage over competitors," it said in a statement.
The Drax group said on its website that the Commission announcement was "in line with expectations" as it proceeded with the conversion of a third generator to biomass fuels.
"Drax welcomes the opportunity to work with the British government and the (EU) to complete the state aid clearance process," it said.
The Drax plant, in the county of North Yorkshire, produces 7.0 to 8.0 per cent of Britain's total electricity, the company says.
Commission state aid investigations are routine across the bloc. If found at fault, member states are required to recover the aid disbursed.
