UPDATE 1-Ad giant WPP lowers full-year expectations again

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LONDON, Oct 31 (Reuters) - WPP, the world's biggest advertising company battling a slowdown in client spending, lowered expectations for full-year organic net sales and profit margin on Tuesday, two months after an earlier downgrade sent shockwaves through the industry.

Led by the high-profile British businessman Martin Sorrell, WPP said it now expected like-for-like net sales growth to come in flat, compared with a previous forecast range of 0 to 1 percent.

It said the headline net sales operating margin was now expected to be flat, compared with a previous forecast of a 0.3 margin point improvement.

"Geographically, like-for-like revenue growth in the third quarter was stronger in the United Kingdom, with all other regions, particularly North America, slipping back," it said.

WPP said its main trading measurement, like-for-like net sales, fell by 1.1 percent in the third quarter, an improvement on the 1.7 percent drop recorded in the second-quarter and a market expectation of a 1.4 percent fall.

Net sales fell by 4.9 percent in North America.

The British group, like its peers Publicis and Interpublic, has been hit by a fall in spending from consumer packaged goods groups, and from fierce competition in the market for online advertising.

(Reporting by Kate Holton, Editing by Paul Sandle)