Radioactive air pollution from the nuclear weapons plant at Coulport, on the Clyde, has more than doubled over the last six years, prompting cancer warnings from campaigners.

Emissions of the radioactive gas, tritium, from the Royal Naval Armaments Depot on Loch Long, have risen steadily between 2018 and 2023 from 1.7 billion to 4.2 billion units of radioactivity, according to the latest official figures.

Campaigners say that tritium is “very hazardous” when it is breathed in, and can increase the risk of cancers. But according to the Scottish Environment Protection Agency (Sepa), the emissions are well within safety limits.

The Ministry of Defence (MoD) has declined to say what has caused the increased pollution. Tritium is known to leak from ageing nuclear submarine reactors, and is also an essential component of nuclear bombs.