* To increase stake in China JV to 80-100 pct in 12-18 months

* To increase China staff to 250 by end-2008, 500 by 2011

* To set up China back-up and service centre this year

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By George Chen

LONDON (Reuters) - Willis Group Holdings Ltd WSH.N, the world's third-largest insurance broker, expects to maintain its annual revenue growth in the China market at at least 20 percent for the next five years and will increase its stake sharply in its local venture, a senior executive said on Friday.

Willis plans to increase its stake in Shanghai-based Willis Insurance Brokers Co, in which it already owns 51 percent, to 80-100 percent and the deal could be completed in the next 12-18 months, said Sarah Turvill, chairwoman of Willis International.

Willis also plans to increase its China staff to 250 by the end of this year from about 200 currently and its total China headcount to 500 by 2011, Turvill told Reuters in an interview at Willis Group's head offices.

"We expect double-digit growth of our revenue in Asia, led by China and India for at least the next five years," she said.