French former presidential candidate Segolene Royal is decrying "political treason" against her after she lost her controversial campaign for a parliamentary seat - quashing her hopes of becoming speaker of the National Assembly.

Royal, a prominent Socialist, is President Francois Hollande's ex-partner and the mother of his four children.

Polling agencies say dissident Socialist Olivier Falorni defeated Royal in Sunday's balloting in the western Charente-Maritime region as part of nationwide legislative elections won by Socialists and their allies.

Her campaign became embroiled in controversy last week after Hollande's current companion - journalist Valerie Trierweiler - expressed support for Falorni on Twitter, seen by some as a show of jealousy.

The defeat deals an embarrassing blow to the Socialist Party, whose hierarchy had backed Royal's candidacy.