A nine-year old boy who became worried about the "regeneration" of his favourite Doctor Who has received a heart-warming Christmas message from actor Peter Capaldi.

Doctor Who superfan David McGilloway, 9, was upset that he would no longer get to see the Glaswegian actor reprise his role in the new series - which will instead feature Jodie Whittaker as the show's first ever female Time Lord.

David's father Brian posted a picture of the letter to his Twitter account which has since received 4,000 retweets and 12,000 likes since it was published earlier today (December 27).

His post read: "#PeterCapaldi is my 9-year-old son's fav #DoctorWho and he was dreading his regeneration. And then he got this letter with some words of comfort from the Doctor himself among his Santa presents. Such a kind man."

Included with the actor's letter was a specially-made Christmas card and a hand-drawn illustration. The message itself is dated from 23 November 1963, the date of the very first Doctor Who television broadcast.

The text from the actor reads: "Dear David, I hope you are enjoying a very merry Xmas. Xmas is always good fun. Well not always. Not every single second. Regenerating is not completely good fun. And it usually happens about Xmas-time.

"But you know what? Even though it can be a little bit icky (like REALLY bad flu) it has always, always turned out good for Doctor Who."

"The new Doctor becomes your favourite Doctor and the one that goes ... well he never really goes, he is always there, somewhere in time and space and if you think about him hard enough you'll see him and he'll see you.

"It's like the doctor says, everything ends and it's always sad. But everything begins again and that's always happy. Be happy.

"So have a brilliant Xmas, a happy New Year and a wonderful life - I'm sure you will."

Social media users have praised the actor's thoughtful gesture towards the young fan.

Domina Willow said: "That's a seriously cool guy thing to do. Good on him for taking the time."

"What a wonderful man he is, he even went back to 1963 to send the letter. Now that's fan service for you! So cool", another wrote.

Another pointed out that Capaldi has a track record of reaching out to his biggest fans.