Staff must provide information on 14 everyday allergens including nuts, milk, celery, gluten, soya and wheat.
The new measures, which come into force on Saturday, cover meals served in bakeries, cafes, care homes and packaged produce sold by supermarkets.
There may be fines for repeat offenders.
According to the European Academy of Allergy, food allergies affect more than 17 million people across Europe.
I had six allergic reactions in the course of a month last year and each time it was because I was told it was fine to eat something that it later turned out I couldn'tOliver Bolland, Allergy sufferer
Some five thousand people need treatment in hospital for severe allergic reactions each year in the UK, and some cases are fatal - causing an average of 10 deaths annually.
Experts say the majority of these deaths and visits to hospital are avoidable, and some are a result of people being given incorrect information about ingredients.
Under the new legislation (EU FIC Food Information for Consumers Regulation), customers must be told if their food contains any of the following:
- celery - including any found in stock cubes and soup
- cereals containing gluten - including spelt, wheat, rye, barley
- crustaceans - eg crabs, lobster, prawns and shrimp paste
- eggs - including food glazed with egg
- lupin - can be found in some types of bread, pastries, pasta
- molluscs - mussels, land snails, squid, also found in oyster sauce
- nuts - for example almonds, hazelnuts, walnuts, macadamia
- peanuts - also found in groundnut oil
- sesame seeds - found in some bread, houmous, tahini
- soya - found in beancurd, edamame beans, tofu
- sulphur dioxide - used as a preservative in dried fruit, meat products, soft drinks, vegetables, alcohol.
Oliver Bolland, 30, from Hertfordshire, is allergic to eggs, fish, shellfish, molluscs and soya.
He said: "My allergies really became a problem when I became an adult - I can't just pop out for a meal with my girlfriend, friends or family.
"I had six allergic reactions in the course of a month last year and each time it was because I was told it was fine to eat something that it later turned out I couldn't.
"Often, waiters don't take my allergies seriously, or they don't know what ingredients are in their dishes.
"I've had to leave important events, including a close friend's wedding, because the waiter didn't check exactly what was in the food and thought I was just being fussy.
"I'll always have to be careful about not accidentally eating something I'm allergic to, but now restaurants and takeaways can no longer say they're not sure whether I can eat something, or that it's probably fine.
"This new law will make a huge difference to my life."
Businesses can choose how they give the information on allergens contained in their food - for example through conversations with customers, leaflets, food labelling or by highlighting ingredients on menus.
But if allergy advice is not clearly given, the Food Standards Agency says there need to be clear signs about where it can be obtained.
Lindsey McManus, from Allergy UK, said: "We hope that restaurants will see the advantage of going this extra mile as it offers huge benefits to the allergic customer and this will only encourage business.
"It will enable people to eat out in confidence, knowing that allergens are monitored in dishes, and that the regulations are being adhered to."
Pre-packaged food bought in supermarkets must also have clear allergen information on the labels.
By Smitha MundasadHealth reporter, BBC News