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The eagerly anticipated sequel to the biggest selling video game in history has gone on sale.

Thousands of gamers are expected to queue at over 400 stores in the UK to get their hands on a copy of Call of Duty: Black Ops.

Special events were held in cities across the globe to mark the release.

Its predecessor, Call of Duty: Modern Warfare 2, generated more than $1bn (£618m) in sales.

That puts it in an elite club of billion-dollar entertainment giants such as James Cameron's Titanic and Michael Jackson's Thriller.

Call of Duty: Black Ops is the seventh game in the series and the third to be developed by US based developer Treyarch.

Piers Harding-Rolls, an analyst with Screen Digest, told ´óÏó´«Ã½ News that he thought Black Ops would do as well, if not better, that Modern Warfare 2.

"We're looking up to 18 million units sold worldwide, putting it in the same league as Modern Warfare 2," he said.

"This edition also has a [Nintendo] Wii version and while the average Wii owner probably won't be that interested, it does mean that the potential market is a bit bigger than before," he added.

Gamers play as a CIA operative or Special Forces agent; members of a clandestine agency tasked with uncovering a Soviet chemical weapon code named Nova-6 during the Cold War.

In addition to standard ground combat, Treyarch have added a mission in which users control a Russian Hind helicopter, as well as flying US spy planes.

There is also a bonus multi player level where users have to defend Washington from waves of flesh-eating zombies.

Call of Duty: Black Ops is the last of the big first-person-shooter titles to be released in the run up Christmas.

Halo Reach - the exclusive XBox 360 title released in September - sold more than 300,000 copies on its launch day, according to the games industry magazine MCV.

Medal of Honor, Call of Duty's traditional rival, has also recently had a refresh.

The latest edition courted controversy by allowing gamers to take on the role of the Taliban, prompting calls from soldiers and politicians for the game to be banned. Its publisher Electronic Arts eventually renamed the enemy forces "the Opposition".

Recent figures suggest that, despite the publicity, Medal of Honour sold fewer than 350,000 units in the UK. By comparison, Call of Duty: Modern Warfare 2 sold an 1.23 million units, according to industry body Elspa.

"Medal of Honour didn't review particularly well and its still the case that those who don't have an average score [in the games press] in the 80 and 90s don't sell as well," said Mr Harding-Rolls.