The prison service is facing a rare sanction known as a crown censure – the maximum sanction possible against a government body – for exposing prisoners and staff to dangerous levels of cancer-causing gas. The Health and Safety Executive (HSE) criticised HM Prison and Probation Service (HMPPS) for failing to manage radon exposure at HMP Dartmoor in Devon, where hundreds were exposed to radon levels above the legal limit. The prison service finally took action to evacuate people at the end of 2023 after repeated warnings, and, by August 2024, had moved all inmates out of the Victorian-age jail. Hundreds of former prisoners and staff have now joined a class action lawsuit against the Ministry of Justice (MoJ), with former prisoners and officers telling The Independent they were left in the dark about their exposure. The HSE announced on Wednesday that it intended to issue the prison service with a crown censure in relation to the scandal. That is the maximum sanction available to the regulator and the equivalent of authorising criminal charges, though no criminal charges will be brought against individuals.The HSE investigation concluded that both prisoners and staff at Dartmoor had been exposed to radon significantly above the legal limit, for a prolonged period of time. Following the decision to issue the censure, a formal hearing will take place to confirm it. The last time the regulator issued such a severe warning was to the Ministry of Defence in 2024, after a staff sergeant was killed when a Jackal vehicle rolled down a steep hill at Catterick Training Area in 2019. HMP Dartmoor closed after high levels of radon were discovered at the jail (PA)The HSE said it was first notified by the prison service of suspected overexposure to radon gas at Dartmoor in February 2023, despite measurements from 2020 showing the prison had unacceptably high levels of the toxic, naturally occurring gas. The government is still paying around £4m a year for the unusable prison after officials signed a 25-year lease in 2022. This includes around £1.5m a year in rent to the Duchy of Cornwall estate, which owns HMP Dartmoor and provides a private income to Prince William. It was revealed last month that the lease has generated at least £2.5m for Prince William so far, and the Duchy has refused to confirm how much tax has been paid on the income. The fiasco has been condemned by the government’s public spending watchdog as a “catastrophic failure”, while civil servants desperate for prison space have been accused of acting in a “blind panic”.Antonia Romeo, who is now the head of the civil service, was the most senior civil servant in the MoJ from 2021 to 2025.The prison service was last issued with a censure in 2020 after nine officers sustained injuries when they were petrol bombed during a training exercise. Radon is a colourless, odourless natural gas, formed when small amounts of uranium in rocks and soils decay. It can build up in indoor areas and, while it can be controlled through ventilation, it is known to increase a person’s risk of lung cancer.More than 1,100 lung cancer deaths are attributed to radon each year in the UK, according to the UK Health Security Agency. The Independent revealed in March that the MoJ is investigating 42 prison and probation sites that have recorded high levels of the gas, including at least 16 prisons. The law firm Kesar and Co has so far taken instructions from people who believe they have been affected by radon at prisons including HMP Exeter, Channings Wood, Lindholme, Portland, The Verne and Dartmoor. A private firm, Sentinel Group Security, has now taken over security at Dartmoor for the initial period of a year, although this could be extended. While prisons minister James Timpson has told MPs that a decision on the future of Dartmoor is yet to be made, insiders said the site has been stripped, locks have been removed from cell doors, and the gas supply to the empty wings has been cut off.A HMPPS spokesperson said: “The wellbeing and safety of staff and prisoners is our priority. Since 2022 we have strengthened radon monitoring and mitigation across the estate. “We have noted the Health and Safety Executive’s decision to begin Crown Censure proceedings, and it would be inappropriate to comment further while these are ongoing.”This article was amended on 15 May 2026. It previously inaccurately reported that the last Crown Censure issued by the HSE was in 2022, however, it was in 2024.