An urgent investigation is under way to find out the extent of the leaks and how to control them so that radioactivity does not reach the water supply.

There has been increasing concern over the state of the tanks and £100m has been spent on a new building to enable them to be emptied. However, this has not yet been completed.

Tests are continuing after the nuclear installations inspectorate revealed that technetium 99 was discovered in a borehole on the site as long ago as last November. Technetium 99 is a byproduct of reprocessing of spent nuclear fuel from two plants which British Nuclear Fuels operates on the site. It remains active for 200,000 years and is known to accumulate in the bodies of shellfish and lobsters.

The NII said in a statement to the New Civil Engineer, which reveals the contamination today, that the quantities so far found posed no public risk but investigations were continuing.

"The NII has requested BNFL's proposals for control of the radioactive material that has entered the ground and expects to receive this soon," an NII statement said.

"BNFL is carrying out further investigations to find out how much technetium 99 is present and if any more substances have leaked into the ground.

"There is no immediate cause for concern. BNFL has been working hard to ensure that there is no hazard to the public," the NII spokesman added.

The NII said the source of the leak was believed to be sludge storage tanks in building B241, which hold about 3,000 tonnes of waste. "These old tanks have been suspected to be leaking for some years and recent modifications have been made to address this."

A November 1998 NII report stated: "The B241 [tanks], constructed in the early 1950s, are considered to be in an unacceptable condition for long term storage. The pre-stressed concrete tanks have shown serious signs of deterioration, including corrosion of reinforcement, cracking of the concrete and seepage."

Work on the building which will enable the tanks to be emptied began in 1998. Emptying is due to start this year but is dependant on NII approval.

The cleaning up of what BNFL calls "historic wastes" is of increasing concern to the government, and the total nuclear liabilities at the site are now estimated to exceed £34bn. Last November the government conceded that BNFL was technically bankrupt since its liabilities exceeded its assets.

The leak will only add to the bill that the taxpayer faces to clean up the wastes, the legacy of the need to produce plutonium for nuclear weapons during the cold war.

For years nothing was done about the tanks but the NII, the government's safety watchdog, has become increasingly edgy about the state of some facilities at Sellafield, particularly when they contain dangerous wastes. It will want the contaminated groundwater recovered but this may prove expensive even if it is technically possible.

Nuclear consultant John Large said: "These buildings are being used well past their design life to store radioactive waste which has outstayed its welcome. BNFL seem always to be fighting a rearguard action."