Neither company was immediately available for comment.

They have increased their prices by more than 100% in several areas since 2021, and have in some cases almost quadrupled them, the report said.

"E.ON and Hansewerk Natur lack the legal prerequisites for the enormous and non-transparent price increases," vzbv board member Ramona Pop was quoted as saying.

She said the district heating market in Germany is not consumer-friendly, lacks competition, freedom of choice and has little price transparency.

"With the two class actions, the vzbv is seeking direct refunds for participating consumers," Pop said, adding customers who believe they are affected can join the class actions free of charge by registering with the Federal Office of Justice.

(Reporting by Maria Martinez and Klaus Lauer; editing by Barbara Lewis)