Sellafield managers have been issued with a formal caution over a leak of radioactive liquid which went unnoticed for 14 months.
The Environment Agency said the leak, which was reported in January 2009, caused no impact to the environment.
But it said it had issued the caution to make sure it did not happen again.
Sellafield said it had made changes to management systems. The leak was spotted on a day when then Prime Minister Gordon Brown visited the site.
The leak of radioactive liquid, in the form of a steady drip, came from a pipe designed to drain condensation from a ventilation duct.
This caused contamination to a small area of ground within the Sellafield site.
It was spotted on 23 January 2009 - the day Mr Brown visited the site to announce possible locations for new nuclear power stations.
In a statement, Sellafield said: "We accept the seriousness of this event and have willingly accepted the Environment Agency's formal caution.
"The Environment Agency has been monitoring our improvements and is happy with the progress being made.
"The condensate was of very low radioactivity, was contained within the site boundary and resulted in no impact to employees, the environment or the local community."
Sellafield said the leak had been about 10 metres (32 ft) from one of the walkways used by staff at the plant.
However it said the area was monitored to the extent that it was certain no employees had been contaminated.
A Sellafield spokesman said they had also put a number of systems in place, such as improving the care of equipment, to reduce the chance of a similar incident recurring.
The Environment Agency said none of the radioactive liquid escaped from the site.
Progress 'under review'
And members of the public were not put at risk as they do not have access to the Sellafield site, it said.
When the leak was discovered, the Environment Agency issued an enforcement notice requiring improved equipment and maintenance routines.
Stuart Page, nuclear regulator at the agency, said: "While the environmental impact of the incident was minimal, a formal caution is warranted to reflect the seriousness with which we view such failings.
"Following the enforcement notice that we issued to the company in July 2009, Sellafield Ltd has made a number of improvements to its management arrangements, and we have confirmed by inspection that these improvements have gone a long way to addressing the failings that we identified.
"More work has still to be completed, and we are keeping the company's progress in these areas under close and frequent review."