Birds kept in less cramped conditions are more likely to be pecked by other birds and thereby catch more diseases, according to the Swedish study reported by The Daily Telegraph.

But campaigners for organic farming said the research was based on old data which only looked at non-battery farmed flocks with higher than normal death rates.

Scientists from the National Veterinary Institute in Sweden looked at the causes of death in 914 birds from 172 free-range, barn-reared and battery flocks.

They found higher death rates in the free-range and barn-reared flocks than in the battery hen flocks.

Most died of bacterial infection, commonly E. coli.

Bacterial infection rates ran at 74 per cent in free-range flocks, 73 per cent in litter-based flocks and 65 per cent in caged flocks.

They reported their findings in the journal, Acta Veterinaria Scandinavica.

But Anna Bassett, a poultry expert at the organic trade group the Soil Association, said the study was flawed because it only looked at poorly managed farms with above average death rates.

She told the Daily Mail newspaper, "It is old data, and the paper itself admits that some of the results may be skewed as this was a time when farmers with no experience of non-cage systems started setting them up and managing them. The paper also suggests that the situation has improved markedly since 2004. We would expect there to be an effect from inexperienced farmers."

Battery farming is due to be banned across the EU by 2012. About 19 million birds are battery farmed in Britain, about two-thirds of the total. Each has the space of about a piece of A4 paper. Studies have shown being kept in such conditions causes them a range of welfare problems, from brittle bones to the prevention of normal behaviour.

Ms Bassett concluded that free-range and organic poultry farming had "the potential to deliver far greater levels of bird welfare," according to the Daily Telegraph report.