LONDON — The owner of an Indian restaurant in Britain with a “cavalier attitude” to food safety was found guilty on Monday of causing the death of a customer who had a severe allergy, after he was served a dish containing peanuts. The owner received a six-year sentence.

It marked the first time in Britain that someone has been convicted of manslaughter over the sale of food.

The customer, Paul Wilson, 38, ordered chicken tikka masala, to go, from the Indian Garden restaurant in North Yorkshire in January 2014, having specified “no nuts” in his order.

Mr. Wilson had diligently avoided peanuts and dishes made with them ever since he had a severe reaction to eating a chocolate bar with peanuts when he was 7 years old. He was “very, very careful,” his mother, Margaret, said, especially when ordering his favorite meal.