REFILE-UPDATE 1-WPP says 2009 sales could be below forecast

(Refiles to correct date in dateline)

* WPP sees 2009 like-for-like sales below previous forecast

* Q1 like-for-like revenues down 5.8 percent

* Operating margins ahead of budget, below 2008

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LONDON, April 28 (Reuters) - British advertising group WPP may cut its full-year forecast after first-quarter like-for-like sales fell 5.8 percent, broadly in line with market expectations.

The world's largest advertising group by revenue had previously said it expected like-for-like revenue to fall 2 percent this year after a 2.7 percent increase in 2008.

"We are in the proces of reviewing our quarter one revised forecasts, but early indications are that like-for-like revenues will be below budget, closer to recent industry forecasts of mid-single digit declines," WPP said in a statement on Tuesday.

Media agency ZenithOptimedia said earlier this month it expected global advertising spending to fall 6.9 percent this year, but WPP's scale and geographical and business diversity help it make the most of such opportunities as still do exist.

WPP said operating margins were ahead of budget but below those of 2008, and said it would be hard to maintain them at last year's levels, after adjusting for the 1.1 billion-pound ($1.6 billion) acquisition of TNS.

The company felt most economic pressure on first-quarter sales in the United States, with Britain less affected. Latin America, Africa and eastern continental Europe still showed like-for-like growth, it said.

WPP said the rate of decline eased in March, although it did not know whether this reflected stabilisation or restocking of inventories.

First-quarter sales were 2.12 billion pounds, an increase of 36 percent including sales from TNS, or 11.1 percent at constant currencies, reflecting the weakness of the pound against the dollar and the euro.

Analysts had expected like-for-like revenues to fall 5.2-7.2 percent, according to a survey of five analysts carried out by the company.

Rival Omnicom (OMC.N), the world's biggest agency by market value, on Monday reported that first-quarter revenue fell 14 percent as firms ditched marketing campaigns and cut budgets. (Reporting by Georgina Prodhan; Editing by Dan Lalor) ($1 = 0.6866 pound)