Some of Britain's biggest fast-food outlets, including McDonald's and KFC, have pledged to make their food more healthy as part of a campaign to cut heart disease and obesity.

Burger King, Wimpy, Nando's and Subway have all promised to cut salt and fat levels over the next year.

Staff will be trained to give diners information on healthy options and ask chefs to cook food without added sauces or salt.

Nando's has promised to make nutritional information public.

The Food Standards Agency (FSA) will monitor the commitments with the aim of making the food eaten by more than three million Britons a day more healthy.

One in six meals is eaten out of the home, a figure that is expected to rise in the next decade.

The FSA intends to announce similar agreements with coffee shop chains, pubs and family restaurant chains early next year.

The commitments are part of a long-term collaboration with the catering sector following a deal with workplace caterers earlier this year.

Rosemary Hignett, head of nutrition at the FSA, said: "Eating out should be fun and we don't want to change that, but we believe restaurants can help make it easier for us to take healthier choices when dining out.

"These companies' commitments, together with the positive work that many have been doing for a number of years, show just how much is possible. We hope that other restaurant chains will be able to emulate this exciting work."

Subway, which has 1300 outlets in the UK and Ireland, will offer free salad with its sub sandwiches as a trial, and cut salt and fat in its best-selling products.

Nando's has pledged to offer fewer products that require a red sign under the traffic-light labelling system and will reveal the nutritional content of its food online.

Burger King will cut salt in its burgers and test a cooking oil with less saturated fat, while Wimpy said it will review salt and sugar levels in its 10 most popular products by next summer.

KFC is to introduce lower-fat mayonnaise and has pledged to meet the FSA's 2010 salt targets for its tortillas and "twister" products by the end of the year.

McDonald's is launching new fruit and vegetable options for children, running trials for 100% fruit smoothies in Happy Meals and offering Big Macs without sauce and fries without salt.