The online poll will run until 28 January 2014, and the duo to lead the campaign will be announced soon afterwards.

The only male candidate is Jose Bove, a French MEP who describes himself as a "farmer of the world" having "milked sheep" for years on a French farm and opposed genetically modified plants and shale gas exploitation in his country.

Monica Frassoni, from the Italian Green Party, is one of the three female candidates who wants to "transform the next European Parliament elections into a real competition."

"We have to convince citizens that they have a say in EU affairs and that, unless they speak up, the EU will split again. If they don't, we will not solve the crisis and our collective irrelevance will be inevitable," Frassoni said.

The other two Green candidates are both from Germany: Rebecca Harms, the current co-leader of the Greens in the European Parliament and Ska Keller, a 31-year old MEP backed by the young Greens.

European elections have had a continuously declining turnout since direct elections began in 1979, with voters tending to think that the EU vote is of little relevance to them.

European parties hope to reverse this trend with trans-national campaigns and lead candidates who run for the top post at the European Commission, a novelty introduced by the Lisbon treaty.

But EU leaders recently poured cold water on hopes that the top candidate of the most popular party will automatically land the commission job.

German Chancellor Angela Merkel said she "sees no automaticity" between the election results and the post.

For his part, EU council chief Herman Van Rompuy warned against parties raising "false expectations."

The Social Democrats meanwhile have opted for a different way of selecting their top candidate, current European Parliament chief Martin Schulz.

Last week, following a vote in the European Party of Socialists (PES), Schulz was nomintated "candidate designate" for the post and is set become the lead candidate following the party's congress in March next year.

On the centre-right, the European People's Party so far has not put any names out and is likely to announce their lead candidate at a party congress in March.