Last checked at 
* Q3 organic net sales growth slows to 2.8 pct
* Says UK market showing signs of Brexit uncertainty
* Says U.S. broadly stable, easing investor concerns
* Shares up 3.2 pct (Adds further CEO quotes, analyst reaction, shares)
By Paul Sandle
LONDON, Oct 31 (Reuters) - WPP, the world's largestadvertising group, reported a slowdown in underlying salesgrowth in the third quarter, saying a weaker performance inBritain perhaps showed the first impact of the country's vote toleave the European Union.
However, the net sales growth - at 2.8 percent - still camein slightly ahead of analysts' expectations, pushing thecompany's shares up 3.2 percent to top the FTSE-100 leaderboard of UK blue-chip stocks.
Chief Executive Martin Sorrell said on Monday the Britishmarket was a "little bit softer, maybe reflecting some of theuncertainty around Brexit" in the quarter, which started daysafter the UK referendum at the end of June.
WPP toned down its forecast for the year, saying bothlike-for-like revenue and net sales for the year would grow by"over 3 percent". It predicted "well over 3 percent" in August.
"Given what's happening in the environment, the slow growth,lack of inflation - with the exception of the UK with thedevaluation of sterling - the focus on costs and the generallevel of uncertainty around the world, I think it's a goodperformance," Sorrell said in an interview.
Net sales growth was 3.8 percent in the first half of theyear.
Analysts said WPP had assuaged some concerns about the U.S.market raised by its rivals in the last month.
Sorrell said the United States was still "pretty strong",with the presidential election having only a small impact.
"The bump from it is small in the context of the overalloperations," he said. "The bigger issue is what happens longterm."
WPP's shares had fallen 7.7 percent over the last month, asinvestors fretted about commentary on the United States from itsrivals.
Third-quarter underlying sales for Publicis grewby just 0.2 percent after it lost a number of large U.S. mediaaccounts in 2015.
U.S. rival Omnicom, meanwhile, said its outlook wasclouded by the election, the likelihood of a U.S. interest raterise before the year-end and the potential effects of Brexit.
Analyst at Citi, who have a "buy" rating on WPP, said thereports from its peers had led investors to question whetherthere was something structurally awry for agency groups.
"WPP's 3Q results should go some way to assuaging theseconcerns," they said, noting that 3.1 percent growth for WPP inNorth America showed a broadly stable trend.
WPP's reported revenue rose 23.4 percent to 3.61 billionpounds ($4.4 billion), reflecting a boost from the fall in thepound.
Like-for-like revenue rose 3.2 percent, down from 4.3percent growth in the first half.
($1 = 0.8208 pounds) (Editing by Sarah Young and Mark Potter)
