An original watercolour illustration has become the most valuable Harry Potter item ever sold - fetching $1.9m (£1.5m) at auction in the US.

The artwork for the first edition of JK Rowling's Harry Potter and the Philosopher's Stone sold for more than three times the expected price.

It was first auctioned in 2001, before the book series was complete, for £85,750 (about $108,000 at current exchange rates).

"This is really the first visualization of Harry and the wizarding world," said Kalika Sands from Sotheby's auction house.

The artwork had been expected to sell for between $400,000 and $600,000, which Sotheby's said was the highest pre-sale estimate for a Harry Potter-related work.

It took nearly 10 minutes for the four-way bidding to finish on Wednesday. The identity of the buyer was not revealed.

The artist behind the illustration, Thomas Taylor, was only 23 years old in 1997 when he created the iconic image of Harry Potter standing in front of the Hogwarts Express - the train that would lead the young bespectacled wizard into the magical world.

It was done using concentrated watercolours with black pencil outlines and took him two days to finish.

Mr Taylor, who grew up in Wales, was one of the first people to read the manuscript for the original Harry Potter book, which went on to sell millions of copies and spawned a lucrative franchise including movies and theme parks.

Ms Sands said the difference in auction price between 2001, when only four of the seven books in the series were published, and now reflected just how popular author Rowling's creation had become.

"In the intervening decades, it's been extraordinary to see just the conclusion of Harry's story, but also how the Harry Potter franchise has taken off, and in that time, new generations have come to appreciate Harry and his journey as well."