Taxpayers will foot the bill for making HMP Dartmoor safe from deadly radon gas – despite the Government paying the Duchy of Cornwall £1.5 million a year to rent the jail, Byline Times can reveal.

This newspaper revealed in January that 96 inmates in two of the six wings of Britain’s oldest jail – owned by Prince William – were being “temporarily” evacuated over fears of poisoning from the gas, which kills 1,000 people annually.

It was later reported that the number had increased to 196 inmates amid work to "permanently reduce" radon levels in the category C prison to ensure staff and prisoner safety.

While a spokesman for the Ministry of Justice (MoJ) was unable to say how much or long it would take to fix the issues, a Freedom of Information request by Byline Times has revealed that the entire bill – expected to be several million pounds – will be paid for by UK taxpayers.

While the Duchy of Cornwall receives a considerable sum from the Government to use the prison – and has a 52,450-hectare estate, mostly in the south-west of England, worth more than £1 billion – it will not contribute to repairs.

A MoJ spokesman said that was not a condition of the lease.

The Duchy of Cornwall did not respond to a request for comment.

Radon is the UK’s second-biggest cause of lung cancer behind smoking. The colourless, odourless, gas is present at the 640-prisoner jail due to the decay of uranium in the granite of its bedrock and walls built using the igneous material.

The MoJ said no inmates or staff have suffered adverse health effects at HMP Dartmoor, which houses a museum attraction in its old dairy, visited by 27,000 tourists a year who pay £4 per adult to enter. It does not turn a profit.

The evacuation follows several years of radon monitoring and comes in spite of the introduction of additional airflow and ventilation measures to combat the problem. Byline Times understands pumps will be installed under the prison in Princetown, Devon, to extract the radon and allow the cells to return to regular use.

HMP Dartmoor was set to close due to its underfunded and crumbling state before a Government U-turn in 2021.

Staff shortages had previously led to prisoners being locked in for up to 23 hours a day, with a lack of capital investment causing “safety and security issues for prisoners and staff”, according to the MoJ.

The MoJ declined to say where prisoners had been moved to, but it is another headache for the beleaguered department, which has overseen a sharp rise in inmate numbers since 1990 – a situation described by Chief Inspector of Prisons Charlie Taylor in December as a “time bomb”.