The report by academics at the University of Bristol's School of Veterinary Sciences, funded by the Morrisons Farming Programme, examined health challenges facing the modern free-range laying hen and identified where improvements could be made.

The academics found signs of stress and health problems could be revealed by the early scrutiny of eggshells. Adjustments to the time at which pullets, hens under a year old, are introduced to the laying farm are also suggested.

Another measure in managing health risks in free-range laying hens is verandas, or winter gardens, for the birds. The glass structures would keep the birds dry and offer sunlight, exercise, reduced stocking density and clean air without the exposure to predators, wild birds and infection.

It also explains that early treatment of birds laying eggs, which show telltale signs of distress on the shell, such as white specks, lumps or ridging, may help avoid long-term ill health and poor productivity.

Dr Claire Weeks, Senior Research Fellow in Animal Welfare at the School of Veterinary Sciences, who led the research, said: "What we found was there is astonishingly little scientific or published evidence-based information on the health of free-range hens. This is all the more surprising given the substantial shift to this production method in Britain in recent years, which is associated with a greater risk of exposure to pathogens and parasites.

"Morrisons are to be congratulated for continuing to invest in poultry welfare on behalf of their farmers, with a view to helping them improve bird health and reduce production costs associated with bird ill health."

Louise Welsh, Morrisons Agriculture Manager, added: "More and more hens are now being raised in free-range conditions, yet to date there’s been a dearth of research activity to address the vital area of hen health.

"This new report has provided what’s arguably the first review of current laying hen health in Britain and offers farmers practical advice on steps that can help them improve bird health and of course business profitability."

Results from the report will be shared with egg farmers who are part of Morrisons’ producer group network and who supply the food retailer with free-range eggs for its own Natures Nest brand.

The latest report builds on Morrisons’ previously commissioned work in poultry welfare, including encouraging hens to range better and how the use of enrichment in broiler chicken sheds can positively impact on bird well being.