Sellafield has pleaded guilty to sending several bags of radioactive waste to the wrong facility, according to the Office for Nuclear Regulation.
The nuclear power company admitted sending four bags of mixed general waste to the Lillyhall landfill site in Workington, Cumbria, in April 2010.
The bags, which contained waste such as plastic, tissues and clothing, should have gone to the low level waste repository, at Drigg.
Sellafield Limited said the error was caused by a new monitor which had passed the bags as "general" waste, making them exempt from strict disposal controls.
All of the original four bags were retrieved from the landfill and returned for correct disposal.
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.
"We have carried out a thorough investigation in partnership with the Office for Nuclear Regulation and have already required Sellafield Ltd to take action to ensure this does not happen again.
"This work of decommissioning and clean-up of the site is a national priority and we will continue to work with Sellafield Ltd, fellow regulators and the Nuclear Decommissioning Authority to ensure that, in meeting this challenge, Sellafield Ltd maintains high standards of environmental performance to protect people and the environment."
Monitoring carried out by the Environment Agency and Sellafield Limited has since confirmed that there was no contamination left at the site or on the landfill equipment.
The company is being prosecuted by the agency and the Office for Nuclear Regulation. Sentencing will take place at Carlisle crown court on 8 March.