The couple are backing the Just Ask campaign

They sampled cheeses and vegetables at Brilliant in west London and discussed food miles and carbon footprints.

Camilla asked for the Southall restaurant's masala recipe and said she would recommend that her food writer son Tom Parker Bowles paid a visit.

The restaurant has signed up to the Just Ask campaign to raise public awareness about the origin of food.

Popular cuisine

Dipna Anand, Brilliant's assistant manager, said: "Both of them were really intrigued to know where the ingredients came from.

"Camilla was asking me how to make the masala and told me she would really like her son to come here."

Organic farmer Richard Harrison said the Prince of Wales was interested in food miles and the carbon footprint of produce.

He said: "He was very interested in the price of milk. He knew it was only 19p a litre about six months ago and that it's up to 30p now.

"He seemed to like the food. He just wished us every luck with the business."

The award-winning restaurant said it sources most of its food from UK producers.

David Fursdon, of the Country Land and Business Association said because of its popularity, Indian food was an obvious area in which to step up their Just Ask campaign.