The CIWF's Peter Stevenson on battery farming methods

According to the study by campaign group Compassion in World Farming, battery hens then suffer fractures and breaks that can lead to death.

The findings come as the European Parliament prepares to vote on the future of intensive poultry farming methods - and ultimately the fate of the UK's approximately 30 million battery hens - on Tuesday and Thursday.

The CIWF report shows that brittle bones - or osteoporosis - are prevalent among hens kept in battery cages and that it can develop after a year of confinement.

'Severe effects'

It estimates that up to eight million hens in the UK go to slaughter with broken bones and up to half-a-million cage deaths could be attributable to brittle bones.

CIWF's Campaigns Director Philip Lymbery said: "Battery cages not only deny hens the freedom to exercise, but also have severe effects on their health too.

"Caging causes the birds' bones to degenerate to the point where they can simply snap - sometimes even killing the bird in the process. The time to scrap this inherently cruel system is long overdue.

"CIWF urges Europe's politicians to ensure that breakfast in the new millennium involves breaking a free-range egg, not a battery hen's leg."

CIWF spokesman Peter Stevenson said that he hoped the European Parliament would "agree to phase out this tremendously cruel rearing system".

Producers are reportedly concerned that such a ban would be disastrously expensive - a claim that Mr Stevenson says the industry's own figures prove is not true.