LONDON (REUTERS) - As Steve Ballmer bows out of Microsoft Corp, the guessing game over who will replace him has started with a British bookmaker putting Nokia's Stephen Elop as the favourite.

Mr Ballmer, 57, unexpectedly announced his retirement last Friday after more than three decades at the world's largest software company, including 13 years as chief executive.

With no heir apparent, Ladbrokes opened up betting on successors for Ballmer who will depart within the next year, with Mr Elop, 49, topping a list of 26 candidates with odds of 5/1.

British and Irish bookmakers offer a wide range of bets as a niche sideline to more lucrative wagers on sports. Online gambling is far more restrictive in the United States.

Mr Elop, a Canadian, led Microsoft's business division before becoming chief executive of the Finnish firm Nokia in 2010 with a brief to revive the once-undisputed leader in mobile phones.

Senior Nokia employees say he has forced them to make faster decisions. But Nokia's ability to compete in the global smartphone market is increasingly questioned; its market share stands at around three per cent, far behind Samsung and Apple which control around 50 per cent between them.

Internal Microsoft candidate Kevin Turner, chief operating officer, is second favourite with odds of 6/1 to replace Mr Ballmer, according to Ladbrokes. In third is former Microsoft executive Steve Sinofsky, who left the company last November.

The top female candidate in the stakes is internal head of devices and studios, Julie Larson-Green, in fourth place.

Microsoft co-founder Bill Gates is ranked as a 50/1 shot to return to fill the void but he is considered more likely than rank 100/1 outsider Tim Cook, CEO at Apple.

Ladbrokes' spokesman Alex Donohue said the market was a "who's who of high fliers" in the technology world.

"With a year to go we anticipate that this market will smash all previous records for technology betting," Mr Donohue said in a statement.