The $35bn (£20bn) merger to create the world's biggest advertising company has collapsed, the companies have confirmed.
In a joint statement, France's Publicis and the US' Omnicom said: "The challenges that still remained to be overcome, in addition to the slow pace of progress, created a level of uncertainty detrimental to the interests of both groups."
Publicis chief executive Maurice Levy told French TV channel BFM: "The decision to discontinue the process was neither pleasant nor an easy one to make, but it was a necessary one."
There will be no termination fee for either company, it was also confirmed.
The proposed merger was announced last July by Mr Levy and John Wren, the boss of Omnicom, to the clinking of champagne flutes in Paris.
The merged group hoped to create a giant able to take on internet behemoths such as Google and Facebook, while making cost efficiencies of nearly £300m.
They were to share their clients and have a greater geographical reach.
But regulatory difficulties in finalising the deal, as well as reported disagreements over crucial appointments such as a chief financial officer, proved too much for the firms.
While waiting for the merger to go through, Omnicom lost key contracts to rival WPP, including those of pharmaceutical giant GlaxoSmithKline and Vodafone.
WPP chief executive Sir Martin Sorrell told the Financial Times that the attempted deal seemed to have been "driven by emotion to knock WPP of its perch and, of course, by French charm".
"In the end it was a case of eyes bigger than tummy," he said.
Investors seemed to have taken the news calmly. Shares in Publicis traded up slightly on Friday as did those of WPP.