The drip was quickly contained, said the company
A radioactive leak at Sellafield went undetected for 14 months, an internal inquiry at the nuclear reprocessing plant has found.
The "steady drip" was detected on 23 January, the day Gordon Brown visited the site to announce possible locations for new nuclear power stations.
Sellafield Ltd said no-one was exposed to the "low radiation levels".
But a spokesman for Cumbrians Opposed to a Radioactive Environment (CORE) said the company should be prosecuted.
The condensate dripped from a line connected to an overhead ventilation duct which serves the Magnox reprocessing plant.
It dripped on to ground made up of a concrete slab and gravelled surface, within the "secure boundary" of the facility.
The leak was initially classified as a level 1 accident on the International Nuclear Event Scale (INES) because of the low level of radiation.
It has since been upgraded to level 2, but only because of the amount of liquid which leaked in the months it lay undiscovered, according to the firm.
INES measurements listed the 1986 Chernobyl disaster as a level 7 incident and Three Mile Island in the United States in 1979 as level 5.
A spokesman for Sellafield Ltd said: "The drip was quickly contained, after which it was sampled and confirmed to be contaminated with radioactive materials.
"Following reassurance checking of employees in the area, work was suspended for the remainder of the day as a precaution but resumed at the start of the following week. One walkway was cordoned off.
"The condensate dripped onto a concrete slab creating a localised area of contamination measuring about 2 sq m - assessments have confirmed that no contamination spread beyond this area, which was around 300m from the nearest site boundary."
CORE spokesman Martin Forwood said: "In the scheme of things we're not talking about a major release of highly-radioactive material.
"But the point is that it had been going on for 14 months, and that's simply unacceptable.
"It's a sign of poor housekeeping and lack of safety inspections."
In 2006, Sellafield Ltd was fined £500,000 over a leak of 83,000 litres of acid containing uranium and plutonium at the plant's Thorp facility.
Carlisle Crown Court heard that the leak, classified as level 3, should have been detected within days rather than the eight months it took.