32,000 First Utility customers will be hit with massive jumps in their energy bills from 1 June.
First Utility — which has 180,000 customers in total, many on fixed-price deals — says the average increase will be 18.6%, but this will depend on several factors, including energy usage.
Affected customers are free to switch and won't be hit by an exit penalty.
This is the latest price hike from energy firms, and follows all the 'big six' energy firms increasing their tariffs by up to 10.8% during the winter.
First Utility founder Darren Braham says: "The network costs, the increasing price of wholesale energy and the social and environmental obligations mandated by the Government have meant that we regretfully need to increase our prices."
Additional reporting by the Press Association