Almost half the population is suffering from common complaints such as exhaustion, colds and migraines because of food intolerance, according to a new report published tomorrow.

"Around 20 million people are suffering from symptoms that impact on their daily lives and yet they are not able to get help from the NHS," said Muriel Simmons, Allergy UK's chief executive.

"We want to see more dietary advice being available and more training given to GPs so that they can recognise that food could be the trigger for some of the symptoms that they are seeing on a daily basis."

While 2 per cent of Britons suffer from a life-threatening food allergy, up to 45 per cent have some form of food intolerance; symptoms include rashes, headaches and stomach pains. Food can also worsen existing conditions such as eczema, irritable bowel syndrome and asthma. The most common food intolerances are to milk and lactose (the sugar in milk), gluten and wheat.

Allergy UK claims that year-long waiting lists to see a dietician are driving sufferers into the hands of alternative practitioners who may have noclinical background or qualifications.

"People can get weird and wacky diets which are very damaging to their health. They start off with one problem and this is compounded by bad dietary advice," said Ms Simmons. She attacks the Government for having done "absolutely nothing".

The charity's "Stolen Lives" report is based on a survey of several thousand people with food problems and reveals that more than a third suffer from more than one symptom - the most common being bloating, diarrhoea and vomiting. One in 10 falls victim to lethargy and anxiety. More than half become so sick that they have to take time off work and two-thirds have problems sleeping. Although 76 per cent regularly go to their GP for help, most said their doctor did not understand their problems.

But a survey of 250 GPs by North Union Healthcare last year showed that while 63 per cent reported an increase in patients reporting a food intolerance, 73 per cent claimed such reports were all in the mind. Food-intolerant celebrities, including Rachel Weisz (wheat) and Victoria Beckham (dairy), were blamed for the surge in self-diagnosis.

The British Dietetic Association's Dr Carina Venter, however, said: "People who consider they suffer from food allergies or intolerances should have access to specialised allergy centres, where their diagnosis could be confirmed or ruled out."

York University researchers report in next month's Nutrition and Food Science Journal, that more than 60 per cent of patients in their study endured years of repeat appointments, suffering and misdiagnosis before food intolerance testing brought relief.

Meanwhile food intolerance has become big business. The value of the "free-from" market for foods devoid of problem ingredients has more than doubled in five years to £138m, says market analysts Mintel.

Case Study: 'Wheat made my head hurt'

Headaches blighted Pauline Napier's adult life until she found she had a wheat intolerance

"I would get a migraine two or three times a month. It used to take me two or three days to recover," says the 41-year-old office administrator from Sidcup, Kent.

"I went down most routes, everything from conventional medication to homeopathy, but nothing seemed to touch it."

A friend persuaded her to be tested for food intolerance. "I didn't think there would be any connection, but I had got to the stage where I would try anything.

"As soon as I got the results I started to cut out wheat from my diet. It was very difficult to start with, because wheat is included in so many things. But it has all been worth it because the migraines have gone. If people look at their diets they may find changes help. It worked for me."