The bags should have been sent to a specialist facility that treats and stores low level radioactive waste.

Carlisle Crown Court heard that a number of significant management and operational failings at Sellafield Limited led to the incorrect disposal of the waste at the Lillyhall landfill site in Workington.

The error was caused by the wrong configuration of a new monitor which passed the bags as ‘general’ waste making them exempt from strict disposal controls.

The bags were retrieved from the landfill and returned to Sellafield for correct disposal.

Monitoring of the landfill site confirmed there was no contamination left there and no harm was caused to the environment or the public.

Ian Parker, Nuclear Regulation Manager for the Environment Agency, said: “Our overriding aim in regulating the nuclear industry is to protect people and the environment from the release of radioactive wastes into the environment.

“While this incident did not lead to any significant harm being caused to the public or to the environment, the failings by Sellafield Ltd that led to the incident were serious and we consider that on this occasion, Sellafield Limited fell well short of the high standards which we expect from them.

“For us, the most important thing is that Sellafield Ltd has learnt the lessons from this and put improvements in place to minimise the chances of this type of incident happening again.”

Ian Barlow from the Office for Nuclear Regulation, said: “We require the nuclear industry to control its hazards and ensure it has effective procedures in place for transporting and disposing of all forms of radioactive material, including waste.

“That hasn’t happened here: a failure in leadership and management resulted in the uncontrolled transport and disposal of low-level waste in the public domain.

“Our decision to prosecute and the £700,000 fine imposed in court today show that this will not be tolerated.

“Where it is necessary to do so, ONR will not hesitate to take enforcement action to ensure the protection of people and society from the hazards of the nuclear industry.”