Segolene Royal, the defeated Socialist in France's presidential poll, says she has split from her partner, Socialist Party leader Francois Hollande.
The announcement ends months of speculation in Paris
Ms Royal announced the end of their 25-year relationship on French radio.
"All couples have their difficulties, and we have had ours... And now we are moving on to a new phase," she said.
Analysts say the Socialists' delight over gains in the parliamentary polls will be overshadowed by the separation of the party's "golden couple".
The BBC's Caroline Wyatt in Paris says the announcement will end the speculation and rumours over the past months about the exact state of their partnership.
Their relationship had clearly been put under strain by disagreements during the presidential election, with Mr Hollande occasionally quoted contradicting Ms Royal's policies, our correspondent says.
"I have asked Francois to live his life his way and he accepted. We no longer live in the same home," Ms Royal told the France Inter radio station.
The revelations are expanded on in a book to be published by Ms Royal this week, in which she says Mr Hollande was having an affair and that she had asked him to leave their home.
In her book, Behind The Scenes Of A Defeat, Ms Royal also says she plans to stand for the Socialist Party leadership in 2008 when Mr Hollande's mandate ends.
The two have been together since they were both at the elite Ecole Nationale d'Administration, which turns out many of France's top civil servants and politicians.
The pair never married but have four children together.