By Ruth Bender and Suzanne Vranica 

France's Publicis Groupe SA tried to reassure investors about the fate of its $35 billion merger with Omnicom Group Inc. of the U.S. on Wednesday, saying that it was confident the proposed new company would get necessary approval to have its fiscal residence in the U.K.

Publicis did note however, that the deal requires French tax approval and the company didn't make any comment about its expectations for such approval.

Publicis' statement, issued late in the day Paris time, came a day after Omnicom CEO John Wren intensified uncertainty about the deal by telling investors on a conference call that he couldn't predict when the deal would close because of its "complexity and open issues." Mr. Wren also said that approvals by various tax authorities were requirements for the deal to close and "there is no Plan B."

Uncertainty about the future of the deal weighed on shares of both companies on Wednesday.

The ad giants took Madison Avenue by surprise last July when announcing their trans-Atlantic merger. Nine months later, with the deal still not completed, investors are perplexed that the two communications giants don't appear to be speaking from the same script.

"We have to admit that we have been surprised by the cautious tone of Omnicom management, especially in contrast with the optimism displayed by Publicis," said Charles Bedouelle from Exane BNP Paribas in a note to investors.

Publicis and Omnicom earlier had blamed the delay on getting antitrust approval from Chinese authorities. Then, last week, Publicis chief executive Maurice Lévy said that the group is still waiting for approval from French tax authorities to exempt shareholders from paying taxes on profits they would make on new shares they will receive during the merger. Mr. Lévy said the delay now means the deal's approval would definitely slip into the third quarter.

He said French tax authorities have told Publicis that there is no reason to speed the approval process as the group is still waiting for China's antitrust green light. In Publicis' statement on Wednesday night, Publicis said the process seeking a tax ruling in France "is standard in this type of merger."

But Omnicom's Mr. Wren sounded more pessimistic on Tuesday, saying tax issues are proving to a bigger obstacle than expected.

Mr. Wren brought to light another tax hurdle: Omnicom and Publicis have applied not only to Dutch tax authorities--as part of the initial plan to incorporate the new Publicis Omnicom Group in the Netherlands--but also to U.K. tax authorities for the merged group to have "exclusive tax residency in the U.K."

Mr. Wren said that they hadn't yet received approval from either authority and that if they failed to do so, it could jeopardize the deal.

In its statement on Wednesday, Publicis said the new company was already registered in the Netherlands. It also said "we are confident" the new company will become a resident of the U.K. for tax purposes and noted "we have no indication that approval" from China "will not be obtained within a reasonable time period."

Talk of the U.K. requirement has taken investors by surprise. "This is the first time they talk about splitting the residency for tax purposes and by extension the first time they say it is a deal breaker," said Keplerch Cheuvreux analyst Conor O' Shea. Mr. O'Shea said he expected a deal to still happen but that the closing would likely slip into the fourth quarter. "The risks of it not happening at all have certainly increased, especially as there are some rumblings about neither party accepting to play second fiddle in the shakeout."

Even so, several executives inside Omnicom and Publicis said they don't think the deal will fall apart. An Omnicom spokesperson said there was no reason to believe that the deal's agreed-upon tax structure couldn't be achieved.

The Dutch Tax and Customs Administration didn't respond to a request for comment. A spokesman said the U.K. tax authority "does not discuss the tax affairs of individual businesses." The French finance ministry and French Treasury didn't return calls seeking comment on the status of the process.

Publicis shares fell 2.7% in Paris trading Wednesday, to EUR61.75 ($85.22), while Omnicom shares were off about 1.6%, to $68.78 in New York.

The U.K., while likely open to the idea of the merged company basing itself fiscally there, "will have to ensure that the businesses are squeaky clean and that the merged entity meets the relevant requirements for being a tax resident," said Ben Jones, a tax expert at Eversheds in London.

The Netherlands is a popular location for multinational companies seeking the reduce their tax bills, considered attractive because of its extensive network of tax treaties and relaxed fiscal treatment of capital gains, dividends and royalty payments. Under pressure to combat tax avoidance, however, the Dutch government in September said it would adopt a tougher approach against "letterbox companies," financial holdings that largely exist for tax purposes and don't have any business in the country, a factor that could complicate an already complex deal structure.

The tax exemption could directly affect Mr. Lévy, who holds a 2.7% stake himself in the French group he has led for more than 30 years, as well as France's Badinter family, Publicis's largest shareholder.

But there are plenty of questions beyond tax. Four months after the deal's initial, optimistic closing date, top executives remain tight-lipped what will happen after the deal's close--particularly who will lead the combined company's media buying operations or be its chief financial officer.

Tom Fairless in Brussels, Maarten van Tartwijk in Amsterdam and Nathalie Tadena in New York contributed to this article.

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