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The game has suffered a 22% drop in sales after last week's record-breaking debut, but that was still not enough to see it fall from the top.
Japanese publishers dominate this week's top five best-selling video games, with Nintendo, Sega, Sony and Konami all enjoying the limelight.
EA Sports had the highest new entry this week, with UEFA Euro 2008 making its debut at number eight.
EA's expansion pack The Sims 2: Kitchen & Bath Interior Design Stuff was the second highest new entry, entering the charts at number 20.
Sega's Superstar Tennis made a return to the top ten following a series of retail promotions, but the highest climber this week is Rayman Raving Rabbids 2, which rose ten places from 32 to 22 after being included in Wii hardware bundles.
The top ten games in full:
1. (1) Mario Kart Wii (Nintendo)
2. (3) Mario & Sonic at the Olympic Games (Sega)
3. (2) Gran Turismo 5: Prologue (Sony)
4. (4) Pro Evolution Soccer 2008 (Konami)
5. (11) Sega's Superstar Tennis (Sega)
6. (5) Tom Clancy's Rainbow Six: Vegas 2 (Ubisoft)
7. (9) Call of Duty 4: Modern Warfare (Activision)
8. (-) UEFA Euro 2008 (EA Sports)
9. (12) Dr Kawashima's Brain Training (Nintendo)
10. (8) Guitar Hero III: Legends of Rock (Activision)
Leisure software charts compiled by Chart Track, (C)2008 ELSPA Ltd
