Illegal levels of food colouring have been found in samples of Indian curry by Surrey trading standards officers.
Pale coloured food indicates natural ingredients
Fifty-eight out of 102 chicken tikka masala samples contained "illegal and potentially dangerous" levels of colourants.
The chemicals have been linked to hyperactivity in children, certain allergies and migraines.
Some countries have banned the colourants, particularly tartrazine and sunset yellow, but the UK allows them.
Food experts and the trading standards officers say there is no need to use the colourings in food.
Diners have been warned that any dish which looks especially vivid should arouse suspicion, as paler colours indicate the use of natural ingredients.
For example, turmeric, saffron and paprika can be used if a red colouring is required.
Trading standards officers launched an investigation, limited to chicken tikka
masala, after uncovering evidence that "worrying" numbers of restaurants were
flouting the law despite earlier education campaigns.
Testing at an independent laboratory found only 44 restaurants were obeying the rules, while one restaurant was using four times the legal limit of colouring
in its curry.
Trading standards now plans to test every curry house in the county.
The Surrey results have worried consumer safety experts who fear a more widespread problem.
"We have evidence to suggest this is a national problem, not just specific to Surrey and we are urging trading standards services across the UK to work with Indian restaurants in their area to ensure the amount of colorants used is within the legal limits," said Phil Thomas from the Trading Standards Institute.
The tests focused on the use of three specific chemicals - tartrazine (E102),
sunset yellow (E110) and ponceau 4R (E124).
The Hyperactive Children's Support Group believes all three are linked to
hyperactivity in children.
Tartrazine, a dye made from coal tar, is used in a variety of cakes, soft drinks and sauces.
But scientists believe it can cause blurred vision and purple skin patches and
is particularly hazardous for asthmatics and anyone allergic to aspirin.
Sunset yellow is used in
juices, sweets and sauces.
Scientists have linked it with chromosome damage and kidney tumours as well as
abdominal pain, hives, nausea and vomiting.
Ponceau 4R, which is illegal in the USA is believed to cause cancer in animals.
Customer expectation was the main reason restaurants used the chemicals, despite natural alternatives, a top curry chef said.
"I'm staggered that so many are using these colourings at these levels,"
said Chad Rahman, the CIEH-FSA National Curry Chef of the Year 2002 and 2003.
"The reason why these restaurants do it is because the customer will say,
`This is not a chicken tikka masala, it's not bright red', and the restaurants
fear they will lose trade.
"Colouring does not enhance the flavour of the food but a lot of people eat
with their eyes."