Barnaby Lenon, chairperson of the British Independent Schools Council, believes many parents have "lost grip" of their children's behaviour and fail to ask them to write the letters.
The former school principal said he had seen a "attrition of manners"' in today's youngsters which means fewer feel compelled to show their gratitude.
An over-reliance on technology has also taken its toll, he added. Many families send a round-robin email or fire off a few text messages, but this lacks a personal touch, he said.
Lenon, who has taught at the famous Eton College, said: "I would be keen for parents to think about the significance of thank you letters. It always has been good manners and it is part of what makes for a civilised society. It adds to the sum of human happiness.
"Everybody says it's dying out? "it's mainly because of a gradual attrition of manners allied to parents losing a grip on their children's behaviour.
"If you ask parents, they will say that many of them were required by their parents to write thank you letters? "But they don't all do that with their own children."
Lenon recommended writing the letters for a range of occasions, from birthdays to dinners and school trips.
Hosts "always remember" and are "incredibly grateful", he said.
Grandparents are often the most disappointed when they do not receive a thank you letter because they were brought up writing them, he added.
"If children get into the habit of doing it, they will continue doing it for the rest of their lives - and will encourage their own children to do the same," the headmaster said.
Daily Mail