The comedy sequel, which sees Bradley Cooper, Zach Galifianakis and Ed Helms's characters head to Bangkok, welcomed impressive ticket sales thanks to a Thursday release and screenings on Bank Holiday Monday.
The Hangover Part II's £10 million total is the second highest debut of 2011, behind Pirates of the Caribbean: On Stranger Tides, and easily outpaces the £3.1 million opening for The Hangover in 2009.
Last week's number one movie On Stranger Tides falls to second spot, Diary of a Wimpy Kid 2 is a new entry at three, Thor slips to number four and Fast & Furious 5 rounds out the top five.
Elsewhere, the extended Bank Holiday break proved fruitful for 3D family animation Rio, which climbs a place to number six.
The mid-week release of X-Men: First Class will provide strong competition for The Hangover Part II, with the Marvel prequel looking to claim the box office top spot this weekend.
The UK box office top ten in full:
1. (-) The Hangover Part II - £10,409,017
2. (1) Pirates of the Caribbean: On Stranger Tides - £4,757,411
3. (-) Diary of a Wimpy Kid 2: Rodrick Rules - £1,476,720
4. (3) Thor - £335,103
5. (2) Fast & Furious 5 - £322,506
6. (7) Rio - £300,865
7. (4) Insidious - £208,313
8. (6) Hanna - £148,339
9. (5) Attack the Block - £142,919
10. (8) Water for Elephants - £129,143
More Hangover coverage:-
> Bradley Cooper, Ed Helms discuss The Hangover Part II
> Watch a behind-the-scenes Hangover II video
> The Muppets spoofs The Hangover II
> The Hangover Part II to be set in Amsterdam
Watch the trailer for The Hangover Part II below: