Sellafield, the UK's most dangerous nuclear plant, has been hacked by Russia and China, it has been claimed.
The plant in Cumbria has come under cyber attack by groups linked to the two nations.
Sellafield, a large multi-function nuclear site on the coast, is also the most hazardous location in the whole of the UK.
The data breach has been covered up by senior officials, The Guardian has reported.
WATCH NOW: The West ‘must wake up’ to China, expert warns
Authorities are uncertain exactly when the plant's IT systems were first compromised, but breaches were first detected in 2015.
Experts noticed that software that is used to attack systems - known as sleeper malware - had been embedded into computer networks at the two-square-mile site.
It is not clear if the software has been removed, raising concerns about the leakage of sensitive and important information.
Some of the plant's most critical activities include moving radioactive waste, monitoring for leaks of dangerous material and checking for fires.
The full extent of data loss is hard to determine due to the site's failure to inform nuclear regulators for several years, it has been claimed.
LATEST DEVELOPMENTS:
The plant in Cumbria has come under cyber attack by groups linked to Russia and China
PA
The discoveries came to light after the Guardian's year-long investigation into cyber hacking was made public today.
The investigation also explored the toxic workplace culture at Sellafield.
Sellafield is used to process nuclear waste from decades of atomic power generation and weapons programmes.
It also hosts the planet's largest store of plutonium.
Sellafield, which is monitored by armed guards, used to be a power station but now processes nuclear waste from across Britain.
Last year, the plant was placed under "special measures" for constant failings in cybersecurity, according to the Office for Nuclear Regulation (ONR) and the security services.
Three individuals are understood to be up for prosecution by the regulator for their cyber failings.
The ONR has declined to comment on any breaches, though did confirm the site had failed to meet cyber standards.
Breaches were first detected in 2015
PA
Sellafield hosts the planet's largest store of plutonium
PA
Sellafield has around 11,000 people working in its 1,300 buildings all connected by 25 miles of roads across 700 acres.
The site is laden with vast quantities of radioactive waste in its silos.
"Entering Sellafield is like arriving in another world: it’s like nuclear Narnia," one senior employee told The Guardian.
"Except you don’t go through a cupboard, you go through checkpoints while police patrol with guns."
A Sellafield Ltd spokesman said in response to the Guardian report: "We take cybersecurity extremely seriously at Sellafield. All of our systems and servers have multiple layers of protection.
"Critical networks that enable us to operate safely are isolated from our general IT network, meaning an attack on our IT system would not penetrate these.
"Over the past 10 years, we have evolved to meet the challenges of the modern world, including a greater focus on cybersecurity.
"We're working closely with our regulator. As a result of the progress we've made, we have an agreed route to step down from 'significantly enhanced' regulation."