Denton Wilde Sapte's plan to create a European super-firm is in doubt because its French target has entered arbitration talks to resolve an internal dispute over the proposed merger.
Sales Vincent & Associes, part of the Denton International Network since 1995, is "split down the middle" over whether to proceed.
It is thought six of Sales Vincent's 12 partners would prefer to leave the firm rather than back a tie-up.
The firm is in arbitration talks at the Conference des Batonniers, the governing body for France's many bar associations, aimed at finding an amicable way to break up the firm.
The dispute will come as a blow to Denton Wilde Sapte, which has made no secret of its ambition to form a pan-European law firm in the next two years.
In 1991, Denton Hall formed Denton International, an alliance of eight continental firms, which is the basis of the proposed European firm. The Scandinavian and Spanish member firms are said to be more enthusiastic for a full merger than the Germans and Austrians.
However, Jacques Sales, head of Sales Vincent, says the break-up will not jeopardise the merger. "We have a dispute with the partners who will not be joining, which will be resolved in the next few months.
"But advanced merger talks with Denton Wilde Sapte are continuing and I believe that whatever agreement we reach will serve as a blueprint for the merger of the other firms in the Denton International Group."
Sales says his firm rejected overtures from a major US firm to merge with Denton Wilde Sapte.
"We have been engaged five years and we will continue," he adds.
Steven Blakeley, strategicmanaging partner at Denton Wilde Sapte, says he sees no reason for the proposed merger to provoke a hostile reaction.
"The firm is successful in its own right but a merged entity can be equally if not more successful," he says.
"The idea that there will be no fall-out is far fetched. But an alliance is always a temporary state of affairs and we expect to move on to the next stage sometime in the next 12 months."
If the merger does go ahead, the new firm is expected to be called Denton Sales Vincent.