SAN FRANCISCO (Reuters) - Facebook Inc will allow users in Britain to wager real money on its service, opening its doors to gambling for the first time as revenue growth slows at the world's No. 1 social network.

On Tuesday, Gamesys, an independent gaming company, launched a version of online Bingo for Facebook users in Britain who are at least 18, and which the company said will pay winners real money. Gamesys also announced plans to offer virtual slots gambling on Facebook.

Facebook confirmed that it is partnering with Gamesys to allow gambling on its service.

"Real money gaming is a popular and well-regulated activity in the UK and we are allowing a partner to offer their games to adult users on the Facebook platform in a safe and controlled manner," Facebook said in a statement.

Facebook spokeswoman Linda Griffin said there are no current plans to offer gambling in any other countries or with any other partners besides Gamesys.

She declined to disclose what portion of the revenue Facebook would take from the gambling on the Gamesys games, but acknowledged that there was a revenue sharing agreement between Facebook and Gamesys.

Facebook takes a 30 percent share of revenue for transactions on non-gambling games on its service, such as when users purchase virtual tractors and seeds in Zynga Inc's popular Farmville game.

Facebook, which makes the majority of its money from online advertising, has seen its revenue growth slow sharply during the past year. In the second quarter, Facebook reported revenue growth of 32 percent, down sharply from more than 100 percent growth it delivered at the same time last year.

Shares of Facebook were down 3 percent at $21.26 on Tuesday afternoon on the Nasdaq.

(Editing by Matthew Lewis)