HMP Dartmoor could remain closed for three years due to the detection of high levels of radon gas, according to reports.

The Prison Officers' Association (POA), the union which represents prison officers, has said it was told to expect the Napoleonic-era prison in Princetown to stay shut until then, the BBC said.

A representative for the Prison Officers' Association (POA) told the BBC that this was the latest estimate from prison service officials “about the time needed to make the jail safe”, as high level exposure to radon can lead to lung cancer.

More than 400 inmates were moved out and up to 300 staff redeployed when the prison was closed last year after high levels of the gas were detected.

The POA representative said she had previously been given a shorter timescale of between 18 months and two years to undertake repairs, although an independent monitoring board said recently that the prison may never open again.

Since the closure, the Grade II-listed building has fallen into general disrepair, leading to a rat infestation and severe damp and mould, according to a report by the prison’s independent monitoring board.

The closure has also had a knock-on effect on local businesses with millions of pounds reportedly having been lost in trade.

Tim Jones, chairman of the South West Business Council, called on the government either to provide the necessary investment to remove all traces of the gas or to declare the building as “unfit for occupancy”.

The 215-year-old building and surrounding land is owned by the Duchy of Cornwall (DoC), which has rented it out to the government since 1850.

The Times newspaper recently revealed that the government signed a new 25-year lease agreement in 2023, which is worth £1.5m a year to the DoC.

Under the terms of the contract, the taxpayer will have to foot the cost of repairs, which could be as high as £68m.