* Q3 sales rise 1.2 pct on organic basis to E2.26 bln
Analysts were expecting revenue of E2.34 bln
* Says environment for clients still "challenging"
* Publicis shares fall (Updates with share price, analyst comment, detail)
By Gwénaëlle Barzic
PARIS, Oct 19 (Reuters) - French advertising agency Publicis , facing fierce new competition with the growth in online advertising, posted third-quarter sales on Thursday that missed market forecasts and sent its shares lower.
The world's third-largest advertising group behind WPP and Omnicom said sales had risen 1.2 percent on a like-for-like basis to 2.264 billion euros ($2.67 billion).
However, this came in below an average forecast for 2.34 billion euros from analysts polled by Reuters.
Publicis's shares slid 5.1 percent to 59.01 euros in early trading, making the stock the worst performer on France's benchmark CAC-40 index.
The French company's relatively disappointing performance followed a sales warning from WPP in August that sent its share price down by more than 10 percent.
But Publicis's new boss Arthur Sadoun took a swipe at WPP for blaming cutbacks in ad spending by consumer goods giants for the woes of advertising companies.
"I think it's a dangerous mistake to blame the problems of our industry on challenges that our customers could be facing," said Sadoun.
Publicis and its peers are under pressure to overcome changes in consumer behaviour, with Internet titans such as Google and Facebook having transformed the industry by using data to better target advertising.
Sadoun who replaced company veteran Maurice Levy in June, approved the strategy initiated by his predecessor to position the group closer to digital consulting, making it compete increasingly with business consultancies such as Accenture and IBM.
Digital consultancy entails advising companies on how their websites and mobile platforms look, and on their Internet advertising and marketing strategies.
Rival Omnicom reported a drop in third-quarter revenue this week, due in part to competition from the likes of Accenture and IBM, although its numbers topped estimates.
However, Sadoun has not endorsed Levy's previous financial performance targets for 2018 which included an operating profit margin of 17.3 to 19.3 percent. Publicis's first-half operating margin stood at 13.2 percent.
The company has already been reorganised over the last 18 months under a project dubbed "The Power of One", aimed at fostering greater cooperation between its myriad agencies.
"Publicis's below-forecast organic revenue growth may check the more positive sentiment on the agencies that had been seen post-Omnicom's Q3 results, which beat expectations in North America," analysts at brokerage Liberum wrote in a note.
Sadoun said the marketplace remained tough.
"What we are doing is difficult, the environment is challenging, but we believe it will result in a sustained organic growth path at Publicis," he said.
($1 = 0.8473 euros) (Writing by Matthias Blamont; Editing by Sudip Kar-Gupta, Greg Mahlich)