A controversial new European study claims free-range hens do not experience major health benefits over birds kept in battery conditions.

The Swedish report found birds kept in free-range or indoor barns suffered from higher rates of infection with mites or bacteria than those kept in cramped, but sterile, battery conditions.

The paper, by the Swedish National Veterinary Institute, found bacterial infections were common in almost three-quarters (74%) of free-range flocks – well above the 65% rate for caged flocks.

However, the report has already attracted criticism from animal rights campaigners, who said the researchers had focused on inexperienced farmers new to free-range farming.

“It is old data [from 2004] 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,” Soil Association poultry expert Anna Bassett told the Daily Mail.

“We would expect there to be an effect from inexperienced farmers,” the paper says. “Free-range and organic systems have the potential to deliver far greater levels of bird welfare but the system is not so controlled or automated as battery cages and a higher degree of stockmanship and management is required.”