Daniel Radcliffe continues to play the film's titular hero
Harry Potter and the Half Blood Prince conjured up £19.7m in its first five days at the UK and Ireland box office.
The film scored the biggest cinema opening of the year so far, including nearly £8m made during preview showings last Wednesday and Thursday.
It dwarfed nearest rival Bruno - last week's number one - which nonetheless had healthy takings of £2.3m.
Bachelor comedy The Hangover climbed back to three, while the latest Transformers movie was at four.
In its three weeks of box office business, Revenge of The Fallen has taken only marginally more than Harry Potter on its debut.
Public Enemies and tear-jerker My Sister's Keeper placed fifth and sixth respectively, while animation Ice Age 3 tumbled from second place to number seven.
The only new entry inside the top 10 was sci-fi flick Moon at eight, its release timed to coincide with the 40th anniversary of the lunar landings.