Three doctors who tested a T-shirt made of identical material to that used for the England team's kit found it offered little protection against ultraviolet radiation.
In the strong sun normal for France in June and July they warn that the players are at risk of sunburn.
Dr Andrew Wright and colleagues from St Luke's Hospital in Bradford shone ultraviolet rays through a layer of the material used in the shirt. They found the loose weave of the shirt allowed 10 to 20 per cent of the radiation through.
Over a three-hour period, a fair-skinned person wearing the shirt would be at greater risk of contracting a sore back and shoulders than one who discarded the shirt and applied sunblock of factor 20-30 instead, they say.
The investigators, who report their findings in a letter to The Lancet, suggest that moisture from humidity or sweat can reduce the sun protection factor further.
Umbro's marketing director, Peter Draper, said: "This is essentially the same kit used by Brazil when they won the World Cup in Los Angeles in mid-July, 1994. It didn't affect them adversely in any way."