Rowling said it was "quite surreal" to be at Buckingham Palace

Speaking at the Queen's 80th birthday party at Buckingham Palace, she said she was "doing well" with the book but did not confirm a publication date.

"You can never really tell till you get near the end," she said.

The mother-of-three read extracts from the most recent Potter adventure at the event, attended by 2,000 children.

But her fans will have to wait a little longer for the as-yet-untitled seventh instalment.

'Surreal'

"I am not quite there yet," she told Reuters press agency. "I am feeling sad as it is the last one. But so far, so good."

Rowling, 40, was joined at Sunday's event by fellow authors Michael Bond, Jacqueline Wilson and Philip Pullman.

The stars of the Harry Potter films were also in attendance alongside Ronnie Corbett, actor Martin Clunes and celebrity chef Jamie Oliver.

Rowling said the garden party was a "fantastic way" to celebrate the Queen's birthday and mark what she described as "a golden age" of children's literature.