The stars of the sixth Harry Potter film were set to walk up the red carpet in London on Tuesday for the world premiere after one of the actors recovered from a bout of swine flu.

Hundreds of fans had gathered in London's Leicester Square despite the rain hours before the premiere of 'Harry Potter and the Half-Blood Prince'.

The pre-release buzz suggests the film will add a sizeable chunk to the $4.5 billion dollars (?3.2 billion) already earned by the Potter movie franchise.

It goes on public release in Europe on July 15, and in the United States and Japan two days later, after being moved back from its original release date of November 2008.

Rupert Grint, who plays Ron Weasley, will attend the premiere after making a swift recovery from a mild case of swine flu last week.

He told journalists on Monday: "It was quite scary when I first found out I had swine flu. I thought 'Am I going to die?' But it was just like any other flu really.

"I had a sore throat and I went to bed for a few days."

Grint will take his place alongside co-stars Daniel Radcliffe, who is the bespectacled Harry, and Emma Watson, better known to fans as the swotty Hermione Granger.

The trio have appeared in all six films so far, after being plucked from obscurity to spend nearly half their lives on the set or preparing for the big screen versions of the popular J.K. Rowling books.

After the red-carpet spectacular, the actors will return to the ongoing filming of the final two movies in the series ? the concluding book "Harry Potter and the Deathly Harrows" was thought too plot-heavy for just one film.

Radcliffe, 19, says he will miss the character with whom he is always likely to be identified when the Potter films come to an end in 2011.

"I've had a fantastic time with Harry Potter, I've loved it. I'll be very sad to leave."

He admitted though that he was looking forward to being able to pick and choose other projects.

"A script will come in and I will not have to say, 'I am busy for the next four years'," he said on Monday.

As the characters grow older, 'The Half-Blood Prince' shows raging teenage hormones stalking the corridors of Hogwarts school while setting the scene for the looming final battle between Harry and his nemesis Lord Voldemort in the concluding films.

Director David Yates, who returns for his second Potter film and is helming the final two as well, said: "For me this film marks a transition. The hormones are starting to fly."

But Watson, who has grown from a shy 10-year-old in the first film to an elegant 19-year-old who models for fashion house Burberry, quashed any thoughts of real romance between the stars.

"We have grown up together from the age of 10 to 19 or 20, and we are just like siblings. There is nothing between the three of us.

"Even among the other cast members there hasn't been anything like that. I'm sorry, it's really boring."

The actors are expected wear white ribbons on their wrists as a mark of respect for murdered cast member Rob Knox, who was stabbed to death outside a bar last year just days after completing the film.