Supporters of French hardline leftist Jean-Luc Melenchon will be urged to vote for Socialist Francois Hollande in a presidential election runoff against incumbent Nicolas Sarkozy, a spokeswoman for the increasingly popular candidate said on Friday.

Clementine Autain also said Melenchon's Left Front aims to secure seats in a left-wing government.

Hollande and Sarkozy top the field of 10 candidates for the first round on April 22 and are likely to go through to a May 6 duel, in which opinion polls suggest Hollande will beat the conservative leader and become the first left-wing president in 17 years.

Melenchon, who quit the Socialist party in 2008, is backed by the Left Front, a coalition of Communists, his own Left Party and others who believe the Socialists have lost their soul. He has leapfrogged into third place in the polls and often draw bigger crowds than either of the frontrunners.

While Melenchon himself rarely says so explicitly, Autain made it clear that the Left Front wanted to capitalise on the former Trotskyist's success and secure a role in government if the left wins.

We will call for a victory over Sarkozy and we will vote for the left-wing candidate who's in front, Autain said.

We want to ensure the left's success. We would like to see the political conditions emerge for us to take part in government, she said in an interview on RMC radio.

Melenchon, who promises among other things a 20 percent rise in the statutory minimum wage and a maximum salary of 360,000 euros a year, has stopped short of stating outright that he would urge his backers to vote for Hollande in a runoff.

Polls show between 70 and 90 percent of his supporters say they would vote for the Socialist if their own candidate is eliminated in the first round as expected.

Melenchon has overtaken far-right candidate Marine Le Pen in several polls as the third-ranked candidate but he remains well short of the scores of 25 to 30 percent that Hollande and Sarkozy are seen taking in the first round.

France's presidential election will be immediately followed by parliamentary elections, which the left last won in 1997, paving the way for five years of government by a Socialist-led coalition that included Communists and environmentalist Greens.

(Reporting By Brian Love; Editing by Paul Taylor)