Oliver led a high-profile campaign for healthier school meals

Oliver said he was "really chuffed" to extend a five-year contract as the face of the chain, appearing in advertising and working with food developers.

The 29-year-old recently led a campaign to improve school dinners.

A British Nutrition Foundation (BNF) spokeswoman said positive role models could improve public eating habits.

Oliver's Channel 4 series Jamie's School Dinners involved submitting to Downing Street a petition, with 271,000 signatures, calling for healthier school meals.

The government subsequently pledged an extra �280m over three years to improve meals in England's schools.

Sainsbury's said that while the programme was on air, the chain saw an increase in sales of its organic produce.

'Community spirited'

Oliver's east London restaurant Fifteen, launched in 2002, trains disadvantaged teenagers as chefs.

The BNF spokeswoman said Oliver "enjoys taking up projects - he is obviously quite community spirited, which is really good".

She added: "There is some research to show that children in particular look to role models and try to mirror some of their behaviour.

"Someone like Jamie Oliver puts a bit more weight behind something - and it gets the media's attention."

But she said one person could not influence a whole nation's eating habits.