APCO Worldwide chief executive Margery Kraus said the firm was "very very close" to sealing the deal with Grey, and insisted that negotiations were unaffected by WPP's agreed takeover.
"It would be 100% inaccurate to say APCO is part of the WPP deal," Kraus said.
While Kraus admitted WPP's bid had delayed negotiations, she said she expected APCO's MBO to be tied up "in a few weeks".
WPP announced its agreement with Grey on Monday. It is buying the group for £734m.
A spokesman for WPP said it had no plans to sell Grey's other major PR firm GCI Group, adding that Grey would be run as a separate entity within the WPP family.
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