The operators of the Sellafield nuclear reprocessing plant in Cumbria are to face a criminal prosecution over the leak of tonnes of radioactive material.
The leak occurred at the Thorp complex at Sellafield
Acid containing 20 tonnes of uranium and 160kg of plutonium spilled from a ruptured pipe into a sealed cell at the site's Thorp complex.
The leak was discovered in April 2005, but investigators claimed it could have happened eight months earlier.
The Health and Safety Executive (HSE) says it is bringing the action.
Operators British Nuclear Group Sellafield Ltd (BNGSL) were strongly criticised after the incident.
No-one was hurt and no radioactive material escaped into the atmosphere.
An investigation by the Nuclear Installations Inspectorate (NII) found "significant deficiencies" in procedures at the site.
Work at the Thorp complex was halted when the leak was discovered.
The HSE alleges BNGSL breached conditions attached to the Sellafield site licence which were granted under the Nuclear Installations Act 1965.
It says the company failed to ensure that safety systems were in good working order and that radioactive material was effectively contained.
A spokesman for British Nuclear Group said: "The company has co-operated fully with the NII throughout its investigation and continues to make good progress against its measures needed to enable the Thorp facility to become operational again.
"As this matter is before the courts it would not be appropriate for us to comment further."