Hundreds of nuclear workers have died of heart attacks and other circulatory illnesses brought on by radiation, a study suggests.

Researchers assessing the risk of worker exposure found that staff at British nuclear plants were more likely to die of heart failure than from cancer caused by occupational radiation.

More than 200 workers at four plants died up to a year earlier than expected because of circulatory problems, while hundreds more are thought to have succumbed to disease after working at other nuclear sites around the country.

Researchers looked at Sellafield, Springfields, Capenhurst and Chapelcross nuclear plants, which employed 65,000 people from 1946 to 2005.

Staff were informed of the research results at special briefings this week before the publication of the study in the International Journal of Epidemiology yesterday.

They were told that there was a clear association between circulatory disease and occupational radiation levels but that it remained to be established if exposure had been the direct cause of early deaths.

Other factors, such as stress, cannot yet be ruled out and researchers said that the ill-effects were more than outweighed by the positive impact on health of being employed rather than out of work.

The study found that the rate of heart attacks and other problems rose in line with levels of exposure to radiation, with workers who endured the highest levels at greatest risk.

Only workers who were exposed to higher levels of radiation before 1980, when new technology was introduced to reduce exposure levels, are thought to be at any significant risk.

Professor Steve Jones, of Westlakes Scientific Consulting, which conducted the research, said: “What we have shown is an association between relatively high levels of occupational exposure to radiation and mortality from circulatory system disease.

“We do see there’s a statistically significant effect. We see a higher mortality for those workers with the highest levels of occupational exposure compared to those with the lowest levels. If there’s a real causal effect here, we could be looking at risks of circulatory disease that are comparable to or even greater than cancer risk.”

Exposure to radiation was measured using the dose badges worn by workers in nuclear plants. Levels of exposure today have been reduced to rates comparable with background radiation.

Doctors still have no clear idea of precisely how radiation might cause long-term harm to blood vessels, but there is some evidence that cell mutations caused by damage to DNA could be involved.

Michael Gillies, a statistician on the study team, said that further research was needed to establish whether radiation was the direct cause of the deaths.

He said: “Other factors associated, for example, with diet, exercise, socio-economic status, shift working and stress may be responsible.

“Many studies associate these factors with an increased risk of circulatory disease and this is clearly something that requires more detailed investigation.”

Professor Dudley Goodhead, of the Medical Research Council radiation and genome stability unit, described the study as a valuable contribution.

But Professor Richard Wakeford, of the University of Manchester, said that the results needed to be treated with caution: “A cause-and-effect relationship cannot be reliably inferred, especially in the absence of a convincing biological explanation, and more research is required.”

Mike Graham, national officer of the union Prospect, said: “Our members have taken some comfort from the finding that generally nuclear workers in the study had lower mortality rates than the local general population.”

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