Martine Aubry, the architect of France's 35-hour work week, has won a ballot for the leadership of the opposition Socialists by the tiny margin of 42 votes.
Supporters of her arch rival, Segolene Royal, immediately contested the result and demanded a re-run, raising the prospect of prolonged feuding within France's main opposition party.
The Socialists said Ms Aubry won 50.02% support in Saturday's ballot against 49.98% for Ms Royal. Valid votes were cast by only 134,784 of the party's 233,000 members.
Ms Royal's lawyer Jean Pierre Mignard said the result was "contested and questionable", while another senior supporter, Manuel Valls, said the vote should be held again.
But Ms Aubry's camp did not appear ready to relinquish victory.
"No one can deny the situation is complicated but no one can deny that Martine Aubry is the new first secretary of the Socialist Party," said her close adviser, Francois Lamy.
"Just because you don't like the result doesn't mean you can change the rules," he said.
The prospect of continued in-fighting and animosity in the ranks of the left is likely to further strengthen the position of French President Nicolas Sarkozy, who has faced little opposition so far in implementing his domestic reform programme.