A number of alleged safety concerns at the Sellafield nuclear site are to be revealed in a BBC Panorama programme.
The BBC said the investigation was prompted by a former senior manager turned whistleblower who was worried about conditions at the site in Cumbria.
It is alleged that parts of the nuclear facility regularly have too few staff to operate safely and that radioactive plutonium and uranium have been stored in plastic bottles.
The company which runs Sellafield has said the site is safe and has been improved with significant investment in recent years, the BBC reported.
The whistleblower is reported to have told the programme his biggest fear was a fire in one of the nuclear waste silos or one of the processing plants.
He told the programme: "If there is a fire there it could generate a plume of radiological waste that will go across Western Europe."
Further allegations in the programme relate to staffing, with the whistleblower saying Sellafield often did not have enough people on duty to meet minimum safety levels.
Responding to the BBC, the head of nuclear safety at Sellafield Dr Rex Strong denied that operating below these levels was dangerous.
He said: "You make alternative arrangements, so the things that have to be done get done. Facilities are shut down if we're not able to operate them in the way that we want to.
"Safety is our priority, and we are managing a very complex site which has got a great deal of hazardous radioactive materials on it."
Sellafield said in a statement given to the BBC that plutonium and uranium samples are "kept securely" and that "to imply that such material is inappropriately managed is simply not true."
:: BBC Panorama: Sellafield's Nuclear Safety Failings was being broadcast on Monday night on BBC1.
Shadow energy secretary Barry Gardiner said: "These revelations about the safety practices at Sellafield must be addressed in detail immediately by the Secretary of State. It is simply unacceptable to breach safe staffing levels.
"This puts the public at an unacceptable level of risk. Sellafield has been heavily criticised by the National Audit Office for its spiralling costs. Shockingly it appears that despite the vast amounts of public money spent there, its safety record is severely lacking.
"It's deeply worrying that these breaches in safety have only come to light from an industry insider. The safety of our nuclear industry should not have to rely on whistleblowers. The Tory Government needs to answer why the inspection regime failed to pick up on this catalogue of failures.
"The Government must carry out a thorough investigation into the safety practices at Sellafield. It must be able to ensure that the public is not being put in danger."