The threat was identified in a new edition of the Building Research Establishment's guidance on radon protection that showed many more areas were contaminated than was previously thought.
New measures will come into effect as part of the Building Regulations on 14 February 2000.
The problem of radon gas seeping up from site subsoil into houses was first identified in the early 1990s.
Trevor Higgs, technical director of the Construction Confederation, whose members include the House Builders' Federation, said: "The newly identified areas of the country that were never thought to contain radon will give local housebuilders a new learning curve."
Maps in the new guide indicate that much larger areas of Wales, the west and south-west Midlands, the Yorkshire Dales and the Lake District may be at risk of radon contamination.
In particular, the whole conurbation of Bristol, stretching from Bath in the east to Weston-super-Mare in the west, and Newport and west Cardiff in South Wales all have a high risk of contamination.
The new maps have been compiled by British Geological Survey and show the extent of radon in underlying geological structure. The maps published in the previous edition of the guidance were derived from radon surveys of existing houses and therefore did not include any evidence from new sites.
Sites will not automatically require radon protection measures but housebuilders will need to seek additional detailed information provided by the British Geological Survey for an extra fee of about £30.
As in the previous edition, the new guidance recommends two levels of radon protection depending on the concentration of the gas. However, the new guidance proposes that a membrane, such as a polyethylene sheet, be used as a first line of defence, with an underfloor sump provided for high levels of radon contamination.
This differs from existing guidance, which recommended that a sump be used for all levels of contamination.
Higgs welcomed the substitution of a sealed membrane for a sump as the first line of defence against radon. "The customer need not have any concern about operating fans to ventilate the sumps," he said.
However, he was concerned that the compulsory installation of a sealed membrane would require "very carefully controlled workmanship" in sealing around services, cavity trays bridging cavity walls, and within suspended concrete floors.
Radon: Guidance on Protective Measures for New Dwellings costs £26 and is available from CRC, 151 Rosebery Avenue, London EC1R 4GB.
