Oldbury nuclear power station near Bristol has stopped generating electricity after 45 years.
Both of the station's reactors were scheduled to be turned off in 2008, but had their operational life extended.
Reactor two was turned off in 2011 and reactor one was finally shut down at 11:00 GMT on Wednesday.
The Nuclear Decommissioning Authority and Magnox, which owns the site, said continued operation was not economically viable.
The switch-off was watched through a video link by many of the station's 500 staff in Oldbury's canteen.
"It'll be a sad day but there's plenty of other things to do," said station director Phil Sprauge.
"The plant has had a number of enhancements over the years, however continued generation is largely down to the excellence of the staff that have operated the plant for those 44 years.
"This fantastic record is one that all staff both past and present, can rightly be proud.
"Today marks a safe and dignified end to the generation of electricity at Oldbury."
Since it was first turned on in 1967, Oldbury has generated 137.5 TWh of electricity which is enough to power a million homes for 20 years, the site's owners said.
Over the next three years the decommissioning process will begin, with remaining fuel from the station's reactor removed and reprocessed.
After this, other hazardous materials from the site will be removed before many of the station's buildings are demolished.
The main reactor building will not be pulled down until about 2100 when the radioactive levels in the building become safe.
Horizon Nuclear Power - a conglomerate formed by E.On and RWE - hopes to build a new power station, next to the existing reactor building, after 2025.