Publicis Groupe SA reported sales that missed analysts estimates while former merger partner Omnicom Group Inc. had the fastest growth in more than two years, underscoring the difficulties facing the French ad company since the collapse of the $35 billion combination.

Publicis shares fell the most since December 2008. The company’s second-quarter revenue declined 1.5 percent to 1.76 billion euros ($2.4 billion), missing the 1.84 billion-euro average estimate of four analysts surveyed by Bloomberg.

Growth was weaker than expected as clients canceled or postponed campaigns and some economies in Europe and emerging countries “lagged,” Publicis said. While business will pick up in the third quarter, the company is “extremely cautious on growth prospects,” it said. Omnicom today said its organic revenue rose 5.8 percent in the period, the biggest jump since the third quarter of 2011, according to data compiled by Bloomberg.

Publicis’s “figures are not satisfactory by our standards,” Chief Executive Officer Maurice Levy said in today’s statement. “They are not consistent with what our operations can achieve.” The Paris-based company will “prioritize cost control in order to achieve a margin closer to our goal for the full year,” he said.

Publicis shares dropped as much as 6.3 percent in Paris. The stock was down 4.5 percent at 56.21 euros as of 4:26 p.m., taking the drop to 15 percent this year. Omnicom rose 0.4 percent to $71.64 in New York.

Omnicom’s sales were led by a 7.9 percent rise in North America. Publicis reported a 1.4 percent fall in revenue from North America, which accounts for almost half of its sales.

Failed Merger

All regions were weaker than expected and ad growth was “particularly poor” in Europe, while North America was weak, Natixis said in a note today, commenting on the Publicis results.

Publicis’s planned merger with Omnicom, which would have created the world’s largest advertising company, was abandoned in May after executives clashed over how to run the combined entity. Tax and regulatory hurdles also created delays, and both companies’ boards voted to end the transaction with no termination fees.

The all-stock deal, billed as a merger of equals, would have created a company with $23 billion in revenue and more financial resources to invest in new advertising technology. While the companies had said the merger would take about six months to complete, the plans bogged down when they couldn’t agree on key management roles, such as who would appoint the chief financial officer.

Revised Plan

Publicis said 2014 will be difficult and its business plan between now and 2018 is being revised to account for market developments and investments to reach its “transformation goals” ahead of schedule.

Digital operations accounted for 41 percent of sales in the first half, up from 37 percent a year earlier. Net income in the period fell to 260 million euros from 313 million euros.

Publicis’s “margins came in much below where we expected at 13 percent versus 13.7 percent in the year-ago period,” said Brian Wieser, a media analyst at Pivotal Research Group in New York. The decline reflects Publicis’s weakest first-half margin result since the first half of 2009, Wieser said.

Omnicom’s first-half net income climbed 7.3 percent to $530.7 million as revenue rose 4.8 percent to $7.37 billion.

Publicis ad agencies won new business during the first half from brands including British Airways, American Express, Renault and Honda.

WPP Plc, the world’s largest advertising company, reported a 1.2 percent increase in revenue for the first five months of 2014, on sales in the U.K., U.S. and fast-growing markets such as Brazil and India.