The live-in lover of French President Francois Hollande today infuriated the mother of his four children by backing her electoral rival for a place in Parliament.
Valerie Trierweiler, 47, has never made any secret of her dislike for Segolene Royal, the 58-year-old who shared Mr Hollande's life for more than 30 years.
Ms Royal was, in turn, devastated when Mr Hollande left her for Ms Trierweiler, a Paris Match magazine journalist, in 2007.
Now Ms Royal, herself a former Socialist government minister who stood for the presidency in 2007, is standing for a place in the National Assembly.
Ms Royal gained 32 percent of the vote in the first round of parliamentary elections on Sunday in the western constituency of La Rochelle, and hopes to win outright in the second round on Sunday.
But Olivier Falorni, a popular local Socialist politician in La Rochelle, is standing against her and achieved 29 percent of the vote last Sunday.
Everybody expected Mr Hollande to back Ms Royal, but today Ms Trierweiler sparked outrage by using her Twitter account to post a message of support for Mr Falorni.
It reads: 'Good luck to Olivier Falorni who was not unworthy, who fights alongside the people of La Rochelle for so many years with selfless commitment.'
Such a message, from the serving first lady of France, could easily swing the election against Ms Royal and effectively end her political career.
'It's a devastating blow for Segolene, and she's infuriated by what has happened,' said a Socialist Party insider.
'The party has been plagued by in-fighting for many years, but Segelone never expected the President's new lover to make such a provocative gesture.'
Asked by reporters, if she was indeed supporting Mr Falorni, Ms Trierweiler simply sent them a text reading: 'Yes'.
A senior Elysee Palace presidential aide meanwhile told Le Monde newspaper: 'I am completely blown away. I expected a government crisis, not a domestic one. It's amazing.'
Ms Royal had no initial comment, but critics said it was another example of Ms Trierweiler abusing her new position at the heart of the French Republic to make mischief.
One of many angry messages which Ms Trierweiler received back on Twitter was from Royal supporter Serge Pastor and reads: 'I did not think that female jealousy went that far.