Britain’s nuclear dump is virtually certain to be eroded by rising sea levels and to contaminate the Cumbrian coast with large amounts of radioactive waste, according to an internal document released by the Environment Agency (EA).

The document suggests that in retrospect it was a mistake to site the Drigg Low-Level Waste Repository (LLWR) on the Cumbrian coast because of its vulnerability to flooding. “It is doubtful whether the location of the LLWR site would be chosen for a new facility for near-surface radioactive waste disposal if the choice were being made now,” it says.

The EA document estimates that the one million cubic metres of radioactive waste disposed of over the last 55 years by the civil and military nuclear industry at the site, near the Sellafield nuclear complex in west Cumbria, is going to start leaking on to the shoreline in “a few hundred to a few thousand years from now”.

The agency voices concerns about “the potential appearance on the beach and in its accessible surroundings, during the process of erosion, of discrete items carrying a significant burden of radioactivity individually”. They could range from tiny particles to larger objects such as hand tools that have become contaminated during use at Britain’s nuclear sites then subsequently disposed of at Drigg, the document says.

Officials at the EA are considering a plan by the companies that run Drigg to dispose of a further 800,000 cubic metres of waste there over the next 100 years. This will include radioactive debris from Britain’s nuclear power stations, nuclear submarines, nuclear weapons, hospitals and universities.

Environmentalists argue that continuing to use the site is “unethical, unsustainable and highly dangerous”. But this is rejected by Drigg’s operators, who describe the risks as “insignificant”.

The EA document, dated 9 January 2014, sets out the agency’s latest assessment of the risks of coastal erosion at Drigg. It was released by the EA this month in response to a request from The Guardian.

Erosion from storms and rising sea levels caused by climate change has “emerged as the expected evolution scenario” for Drigg, it says. Experts have concluded that this is almost bound to happen.

Although Drigg was meant to be for low-level radioactive waste, there are fears that some of the disposals in the past may have included higher-level wastes. The rest of the nuclear industry’s medium and high-level wastes are still awaiting an agreed disposal route, with successive UK governments failing for decades to find a deep burial site.

The site, which covers about 110 hectares, is between five and 20 metres above sea level. It is run by a consortium led by the US engineering company URS, the French state-owned nuclear company Areva, and the Swedish nuclear firm Studsvik. The consortium has already been asked by the EA to look at options for improved flood defences.

According to Ian Parker, the EA’s nuclear regulation group manager in Cumbria, the agency had reached its latest conclusions after detailed technical assessments. “It’s highly probable the coast will erode and the waste will be disrupted,” he said.

The EA was taking “a very conservative approach” to reduce risks to future generations, he argued. Further public consultations on the proposal to keep using the site were due in the autumn, and no final decision would be taken until next year.

Drigg’s operator, LLW Repository Ltd, said it had introduced new restrictions on the amounts of radioactivity that can be disposed of at the site in order to make sure that radiation doses to people will be “very small” if the wastes are exposed by coastal erosion.

The company’s head of science and engineering, Dr Richard Cummings, accepted that erosion could start “in a few hundred years”. But he added: “The radioactivity in the wastes will largely have decayed away by this time.”

Carrying on disposing of waste at Drigg was sustainable and ethical because future generations would be given the same protection as now, Cummings said. “The stringent regulatory requirements we have to meet ensure that even if people in the future forget about the repository and the wastes disposed there, the effects will be insignificant.”

But Martin Forward, from Cumbrians Opposed to a Radioactive Environment, pointed out that more than 1,200 radioactive particles from Sellafield had been found on nearby beaches in recent years. “The potent threat of rising sea levels makes the future use of the site unsustainable, unethical and highly dangerous for future generations,” he said.