Parents who fear their children may have an allergy should take them to their GP rather than buy tests online, NICE has warned.
The National Institute for Health and Clinical Excellence (NICE) has issued new guidance aimed at speeding up the NHS diagnosis of food allergies in children after it found many tests which can be bought online put children at risk.
Experts from the watchdog said there was no scientific basis for many of the tests and that they could leave children malnourished by urging parents to put them on unnecessarily strict diets.
The watchdog urged parents to avoid costly private or alternative testing and to see their GP for a referral to an allergy specialist if necessary.
Dr Adam Fox, a consultant in paediatric allergy at Guy's and St Thomas' Hospital in London, worked on the guidelines.
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."
He said the more professional the website and the more "pseudo science" it contained, the more patients were taken in.
"They have their children on sometimes very restrictive diets which are sometimes completely unnecessary."
The guidelines, aimed at those in primary care working with children and young people with suspected food allergy, says parents who opt for private allergy tests should ensure they are seeing a reputable, trained specialist.