Harry Potter and the Order of the Phoenix has broken the UK box-office record for the best four-day opening, according to Warner Bros.
The studio behind the franchise says Phoenix has made £16.5m, partly due to the fact that it is currently the school holidays.
The previous record holder, Star Wars Episode III: Revenge of the Sith, made £14.3m two years ago.
"Our Wednesday opening was a great move," said Warner Bros domestic distribution president Dan Fellman. "We've never had the ability before to open a Potter movie when all the kids were out of school. We certainly have the potential now for Phoenix to become one of the largest-grossing [in the franchise]."
In the fifth film in JK Rowling's series about the boy wizard, Harry leads a band of rebels against a usurper headmistress determined to ban pupils at Hogwarts from casting spells to defend themselves against the resurgent dark wizard Voldemort and his followers.
The film is also currently number one in the US, where it ousted Transformers from the top spot at the weekend with a $140m (£68.75m) haul.
The sixth Harry Potter film, Harry Potter and the Half-Blood Prince, is currently being filmed, while the seventh book, Harry Potter and the Deathly Hallows, is released on July 21.