The Didcot B power station in Oxfordshire has partially resumed generation from a section of the plant devastated by a fire last week.
The blaze knocked one of two modules at the 1.4GW combined cycle gas turbine plant out of action, but it is now back online and operating at 50 per cent capacity, owner RWE said.
The damage is still being assessed and, while some more capacity may be restored in coming weeks, it is not known when repairs will be complete and the module will be restored to full service, a spokeswoman added.
No-one was injured during the incident, but German-owned RWE said in a statement that investigations continue into the cause of the fire that damaged the cooling tower at Didcot's Module 5. The blaze caused no damage to the rest of the plant and Module 6 continued to operate at full capacity.
Roger Miesen, head of hard coal and gas at RWE Generation, said: "It's good news that Module 5 is back online so quickly. It will be available to generate power this winter and essential repairs will be in the region of single-digit million pounds.
"Didcot B, as part of RWE Generation's wider fleet, has an important part to play in contributing to the UK's security of supply. Our thanks go out for the fantastic response and dedication of the emergency services who worked alongside RWE teams to bring the fire under control quickly and safely.
"Huge credit is also due to the RWE site and central engineering teams whose expertise meant that we are able to bring Module 5 back into service after only nine days."
The news comes a day after a National Grid report showed a series of recent fires and closures of power stations had increased the prospect of an energy crunch this winter and that unusually bad weather would see the UK struggle to keep the lights on.
The winter outlook report had already factored in the prospect of the affected module at Didcot returning to service this week.