Europe to vote on chicken and egg issue.
The claims are in a report by Compassion in World Farming, released as the European Parliament readies itself for a vote on the future of battery hen farms.
The group says the farms deny hens the exercise necessary to keep them healthy and in the UK about 30 million hens are kept in battery cages.
Around 500,000 of the deaths of hens in cages could be attributed to brittle bones, a condition the group believes is due to osteoporosis. One study of caged hens found that all the birds were osteoporotic after a year of confinement.
More than eight million hens a year could be arriving for slaughter having suffered broken bones. Scientists found that an average 29 per cent of live hens for the slaughterhouse had at least one freshly-broken bone.
Often a slow death, osteoporosis can cause paralysis and starvation at the back of the cage.
The hens' bones can become "so brittle that the ribs give way causing the heart to be punctured," according to a poultry adviser quoted in the report.
CIWF's campaigns director Mr Philip Lymbery called for the farming to be stopped, saying: "Caging causes the birds' bones to degenerate to the point where they can simply snap, sometimes even killing the bird in the process."
The EU Parliament is expected to vote on proposals for a new laying hens Directive in Brussels today.