Chocolate bar and ice cream wrappers containing latex can trigger potentially fatal allergic reactions in sensitive people but there is no law for it to be listed on labels, experts have warned.

A study commissioned by the Food Standards Agency found that one-third of packaging tested was contaminated with latex, which in some cases transferred to the food.

In one make of chocolate biscuit, the amount was 20 times the minimum necessary to produce a reaction. High levels were also found in an ice cream.

Professor Barry Kay, an allergy expert at the Royal Brompton Hospital in London, said: "Latex can give contact hypersensitivity ... and so these individuals should avoid touching contaminated packages. There should be legislation on latex used in food packaging."

Graham Lowe, an expert adviser to the UK Latex Allergy Support Group said: "For a few people, natural rubber latex is a very potent allergen and for [them] there is no safe level of exposure."

There is no agreement on a safe level of latex, but some experts say a billionth of a gram is enough to cause a reaction in sensitive people.

Natural latex, derived from rubber trees, is used in meat netting, fruit and vegetable stickers, rubber bands and confectionery wrappers.

Mr Lowe said the transfer of latex proteins to food could account for otherwise inexplicable reactions in some people.

There have been two recent reports of people reacting to latex in chocolate bars, according to Chemistry & Industry magazine. In one case, a woman developed a rash around her mouth.

Proteins in the latex which migrate from the packaging to the food are destroyed by heat. But there is a problem with "cold sealing", the technique used to wrap products such as chocolate and ice cream.

The Food Standards Agency said it was too early to draw a firm conclusion, but advised people not to alter their eating habits.