Bosses at the Sellafield nuclear reprocessing plant in Cumbria have been ordered to make improvements following a leak of radioactive material.
The leak is thought to have begun last August
Acid containing 20 tonnes of uranium and 160kg of plutonium spilled from a ruptured pipe into a sealed cell earlier this year.
An ongoing investigation by the Nuclear Installations Inspectorate (NII) has found "significant deficiencies".
It has issued two improvement notices which must be acted upon by October.
Work at the Thorp complex was halted when the leak, which could have occurred as long ago as August 2004, was discovered in April.
Investigations are focusing in part on how long the leak had lain undetected and reliability of monitoring systems.
A clean-up operation is continuing and similar pipe work elsewhere in the plant has been checked.
Sellafield's managing director, Barry Snelson, has admitted that the plant may remain closed for months.
Now, the NII says that during initial investigations it has found evidence of "significant deficiencies" and has issued two improvement notices.
The watchdog says these do not prevent further enforcement action when more investigations have been completed.
The British Nuclear Group, which operates Sellafield, has until 8 October to take the necessary action. This includes improving leak detection, and making sure the detection equipment is fully maintained.
A NII spokesman said the organisation wanted to ensure management took "prompt action to prevent similar events recurring".
The inspectorate says it has not ruled out legal action over the leak, which managers say was contained.