Parents could be putting their children at risk by buying allergy tests online and on the high street, experts have warned.

Youngsters can end up malnourished if they are put on restrictive diets and there is no scientific basis for many of the tests, according to experts from the National Institute for Health and Clinical Excellence.

The watchdog is urging parents to avoid costly private or alternative testing and see their GP for a referral to an allergy specialist if necessary.

It has issued new guidelines aimed at speeding up the NHS diagnosis of food allergies in children.

Adam Fox, a consultant in paediatric allergy at Guy’s and St Thomas’ Hospital in London, worked on the guideline.

He said experts did not know exactly why the number of adults and children diagnosed with allergies was rising across the UK.

One in 20 children has a food allergy, according to a large-scale UK study, with one in 50 allergic to nuts.

He said doctors frequently saw parents who had taken their children for private testing. “It’s not at all unusual that parents will come in with their print out of things they have been told to exclude based on completely invalidated types of testing.

“The more slick the website, the more they are likely to believe it.”

Dr Fox said the youngsters were often suffering from underlying allergies that did indeed require attention, but they had got “lost” in a long list of other things that had emerged from private testing.

High street and online tests were both “useless” and were “getting in the way” of clinicians finding out what was really going on. However, he acknowledged that a “main driver” of parents seeking out alternative tests could be the sometimes long wait to see a specialist.

Nice made a specific warning over some allergy tests sold on the high street and over the internet, including the Vega test, kinesiology and hair analysis.

Some youngsters also suffer gastrointestinal problems such as vomiting, nausea or constipation.

Data suggests that up to 20 per cent of children who are reported as having a food allergy do not have a confirmed diagnosis, despite frequently being on restricted diets.

Factbox

Children and young people are most commonly allergic to:

• cow’s milk
• fish and shellfish
• hen’s eggs
• peanuts
• tree nuts
• sesame
• soy
• wheat
• kiwi fruit

Food allergy can take the form of a sudden severe reaction (anaphylaxis) or a more long-term response, such as the development of skin conditions including eczema or itchy rash.