Record-breaking levels of naturally occurring radioactive gas have been found in two Cornish homes.
Radon is a colourless and odourless gas
The concentrations of radon are thought to be up to 85 times higher than recognised safety limits.
Both properties are in the Kerrier district. One is a privately-owned house and the other a rented flat.
Each occupier has been informed and warned of the health risks they face and are being advised on what remedial action can be taken.
Radon tests are to be offered to neighbouring householders.
Radon is a natural radioactive gas that enters buildings from the ground and exposure to high levels over extended periods of time leads to an increased risk of lung cancer.
It is estimated that about 2,500 cases of lung cancer are caused by indoor radon each year in the UK.
Levels of 17,000 bequerels of radon per cubic metre of air were detected in one property and 12,000 in the other.
The highest value previously found in Britain was 10,000 bequerels per cubic metre.
Both are well above the "Action Level" set by the National Radiological Protection Board (NRPB) of 200 bequerels.
The radiation watchdog said: "The higher the concentration, the greater the risk to health and the earlier remedial action should be taken.
"When the concentrations are very high, as in these two cases, the NRPB strongly advises that effective action is taken as soon as is practicable and certainly
within a few months."
The average radon level in British homes is 20 bequerels per cubic metre.
The high levels in the Kerrier district were discovered through the Radon Roll-out Programme, a Department for Environment, Food and Rural Affairs
(Defra) initiative to identify and make safe radon hot-spots.
Installations designed to protect homes from radon typically cost between £500 and £1,500.
Common systems involve creating an impermeable barrier between the earth and the property and venting the gas.