A study by the epidemiologist John Urquhart, to be presented at a conference at City Hall in London marking the 20th anniversary of the disaster, suggests that infant deaths may have risen by 11 per cent between 1986 and 1989 in those areas compared with 4 per cent in other areas, a correlation that Mr Urquhart describes as very significant.

Mr Urquhart - the author of a previous study which suggested that 2,000 more children than normal died before their first birthday between 1986 and 1989 - obtained infant death figures from 1983 to 1992 for 200 hospital districts across Britain. Areas across which cloud passed such as Liverpool, Bradford, Leicestershire, and Bristol, showed higher than average infant mortality which, he suggests, cannot entirely be explained by social factors.

The study also suggests that a downwards infant mortality trend was interrupted in the four years after the disaster at the Ukrainian power station and continued to rise until 1992 in the most contaminated areas.

Mr Urquhart argues that a plume of fallout from Chernobyl arrived near the Isle of Wight and passed over Bristol into south Wales. Another plume clipped the coast of Kent and then covered most of East Anglia and part of Essex. Another worked its way from east London to Hertfordshire, resurfacing in parts of Northamptonshire and Leicestershire.

Parts of West Yorkshire and most of the West Midlands, Wales, Merseyside, Lancashire, and Cumbria were significantly affected.

Mr Urquhart, who gave evidence in the 1980s to the Government investigation led by Sir Douglas Black into evidence of a leukaemia cluster near Sellafield, Cumbria, said: "Previous research has established that there has been an increase in thyroid cancers in the young in the north of England for which Chernobyl is the probable cause.

"This new study shows that the infant mortality trend, which was otherwise downwards, rose for a period of four years in England and Wales after Chernobyl. The results based on such a large population suggest that the effect of radioactive fallout could be two orders of magnitude greater than previously suspected."