BERLIN (Reuters) - Support for Germany's opposition Greens is at its highest level this year after their new leaders made a pitch for conservative voters ahead of next year's federal elections, an opinion poll showed on Wednesday.

The Forsa poll, which coincides with increased media speculation about a possible future coalition between the Greens and Chancellor Angela Merkel's conservatives, put the Greens at 16 percent, up three percentage points from the previous survey.

More than one in three Germans would welcome such a coalition, said the poll, published in weekly Stern magazine.

Women and younger voters were particularly positive about such a coalition.

At a three-day party congress last weekend, Greens including Katrin Goering-Eckhart, a newly elected party leader and prominent Lutheran, made clear their ambition to lure voters away from Merkel in the countdown to next September's election.

Merkel's conservatives were down three percentage points at 36 percent in the Forsa poll, though they remain Germany's most popular party, ahead of their main rival, the Social Democrats (SPD). They were unchanged from the previous week at 26 percent.

The poll put Merkel's current junior coalition partner, the pro-business Free Democrats (FDP), still at four percent, below the five percent threshold needed to win seats in parliament.

The Left Party would get eight percent and the maverick Pirates, who argue for more freedom on the Web and find support mainly among younger voters, would scrape into parliament with five percent, the poll showed.

If the poll results were repeated in the election, political analysts say the most likely outcome would be another unwieldy 'grand coalition' of Merkel's conservatives and the SPD, like the one she led from 2005 to 2009.

But with the SPD's candidate for chancellor, Peer Steinbrueck, ruling out such a deal, a coalition between Merkel and the Greens would also be an option.

The SPD and the Greens say they want to form a coalition and end Merkel's rule but Wednesday's poll results suggest they would fall short of a majority.

(Reporting by Gareth Jones; Editing by Silvia Antonioli)