BEIJING, Dec 1 (Reuters) - Growth in China's factory sector slowed more than expected in November, a government study showed on Monday, underlining the challenges facing the sector as manufacturers fight rising costs and softening demand in a cooling economy.

The official Purchasing Managers' Index (PMI) slipped to 50.3 in November from October's 50.8, but remained above the 50-point level that separates growth from contraction on a monthly basis.

Analysts polled by Reuters had forecast a reading of 50.6.

China's economy grew 7.3 percent in the third quarter of this year, its slowest pace since the global financial crisis, and risks missing its official annual target for the first time in 15 years, adding to concerns the world's second-largest economy is becoming a drag on global growth.

After months of more modest stimulus measures, China cut interest rates unexpectedly on Nov. 21, stepping up efforts to support the economy as it heads towards its slowest expansion in nearly a quarter of a century, saddled under a mountain of debt.

(Reporting by Judy Hua and Koh Gui Qing; Editing by Kim Coghill)