The leak is thought to have began last August

The Thermal Oxide Reprocessing Plant (Thorp) stopped production in April when the leak, which went undetected for up to eight months, was discovered.

Sellafield's managing director, Barry Snelson, admitted to the BBC that the plant may remain closed for months.

Safety regulators claim the discharge could result in criminal charges.

Mr Snelson described the incident as "a stumble, not a fall".

The accident happened when a narrow pipe fractured, spewing nitric acid onto the floor of a concrete-lined cell in the Thorp reprocessing complex.

The acid contained 20 tonnes of uranium and 160kg of plutonium.

It is thought the pipe may have fractured in August, but the leak was not discovered until eight months later due to a combination of a faulty gauge and human error.

No staff were contaminated.

Last week, Sellafield was told to improve the way it discharged low level radioactive waste water into the Irish Sea.

Environment Agency inspectors issued an enforcement notice after finding its filtering system needed to be improved.

Operators British Nuclear Group said no discharge limits had been breached and it was "committed" to improvements.