The National Radiological Protection Board added Northamptonshire, and parts of Somerset and Derbyshire, to Devon and Cornwall as areas where at least 1 per cent of houses suffer concentrations of radon above the official action level.
However, a spokeswoman for the board said yesterday that although the affected areas had been designated on a county basis, 'radon is no respecter of county boundaries'; for example, the geological structures giving rise to radon in Derbyshire continue on towards Sheffield.
'It's fairly obvious that there will be a percentage of houses affected in the outskirts of Sheffield,' she said.
The radon problem is less severe than that found in Devon and Cornwall, which, in 1990, were the first counties to be designated affected areas.
Radon seeps up through the soil from small concentrations of uranium ore in the rocks beneath and collects in basements or in the spaces beneath floors.
The underlying rock in the South-west is fractured granite which has comparatively large deposits of uranium.
But although the rock underlying Derbyshire is limestone, it contains enough uranium to pose a radon problem.
Householders in the affected areas should write to the board to have radon concentrations in their houses measured individually. 'One house can be OK and the next have high radon levels,' the board said.
The incidence of high radon concentrations can be patchy, with perhaps only one house in a single street having radon that exceeds the recommended action levels.
The Government has not moved to help house owners affected by the problem.
The only funds available are means-tested local authority grants for remedial work. With the squeeze on local authority spending, few such grants are being made.
In Devon and Cornwall the local building regulations were altered so that the construction of new houses took account of the prevalence of radon.
Most existing houses can be modified to minimise the radon concentrations - by putting in ducts or fans to extract the gas - for less than pounds 1,000.
Paradoxically, despite the high radon levels in South-western homes, the incidence of lung cancer in Devon and Cornwall is less than the national average.
However, cancer epidemiologists believe this reflects the rural character of the two counties and recent demographic patterns. Older people move to the area on retirement and younger people move away in search of work before receiving sufficient radon exposure to produce a noticeable statistical effect.
Attempts to trace the effects of radon directly are complicated by cancer resulting from cigarette smoking.
Together with the Imperial Cancer Research Fund, the board is investigating more than 600 victims of lung cancer in Devon and Cornwall to see if they have a history of radon exposure.