Radioactive water vapour was released into the atmosphere during an incident at Hartlepool Nuclear Power Station last week, it has emerged.
British Energy declared a site incident on 9 June after a leak on a pipe carrying radioactive by-products.
At the time, British Energy said the leak of contaminated water had been contained within the site.
The Environment Agency now say there was a release of radioactive vapour, but stressed it was a very low level.
The incident happened after water carrying radioactive by-products was released into the reactor building.
A spokesman for British Energy said they are allowed to discharge a certain amount of material into the atmosphere and, because this limit was not exceeded, it was left to the Environment Agency to inform the public.
He said: "In accordance with Environment Agency regulations water vapour was discharged through our designed facilities in line with normal station processes.
"As this was within the authorised discharged limits our usual reporting process is through the Environment Agency.
"Close liaison has been maintained with the Environment Agency throughout the event."
However, Hartlepool councillor Geoff Lilley has expressed concern over the time taken to inform the public about the radioactive vapour.
He said: "What we find of great concern is when British Energy tell us there have been no emissions into the atmosphere and then later on we find out that there have been.
"It's about trust. It's about representing the ward next to a nuclear power station and knowing that statements made are accurate and that we can trust British Energy."
'Wanted to be sure'
A spokesman from the Environment Agency defended the delay on reporting the atmospheric emissions.
He said: "We knew very quickly that the release posed no significant risk to the public and we didn't want to release provisional information while the recovery operation was going on.
"We wanted to be sure of the data and be sure of our interpretation before giving information to the public.
"From this incident, the maximum exposure to radioactivity someone could have suffered was two microsieverts. On average, a person absorbs around 2,200 microsieverts each year. Just flying to Spain and back, you absorb around 20 microsieverts."