Rowling, who has earned a £560 million fortune from the seven Potter stories which sold more than 375 million books worldwide, sets her story three years before Harry is born and features the characters Sirius Black and James Potter, Harry’s father. They get in a scrape with irate policemen before making their escape via broomsticks, drumsticks and a little bit of magic.

“I had great fun writing the card and I’m absolutely delighted that it has raised £25,000 for two such worthy causes,” Rowling said.

The author, whose books have also been turned into a series of record-breaking films, has previously stated that if she did write another Potter tale, it would not be for another ten years.