BERLIN (Reuters) - Support for Germany's Free Democrats (FDP), the junior partner in Chancellor Angela Merkel's center-right coalition, has reached the 5 percent threshold needed to enter parliament for the first time since October, a poll showed on Sunday.

The Emnid survey showed support for the liberal, pro-business FDP - a veteran participant in German coalitions since World War Two - nudged up from 4 percent the previous week.

The FDP gained a record 14.6 percent of the vote in Germany's 2009 federal election but support has slumped to as low as 2 percent since then due to broken election promises and infighting, much of which has focused on unpopular leader Philipp Roesler, who failed in his attempt to inject new dynamism after taking over as head of the party in May 2011.

But criticism of Roesler has waned somewhat after the FDP took 9.9 percent of a January vote in the Lower Saxony region, defying forecasts that it would be ejected from the state assembly.

The party also seems to have shrugged off an uproar last month surrounding a female journalist's allegations that Rainer Bruederle, its parliamentary leader, told her she could "really fill a dirndl" - a traditional low-cut Bavarian dress.

The FDP is due to hold a party congress next weekend, when it is expected to confirm that Bruederle will lead its campaign in the federal election in September, while Roesler is expected to be re-elected as party leader.

The survey gave Merkel's conservatives 40 percent, down 1 percentage point compared with the previous week but still by far the most popular party.

The main opposition Social Democrats (SPD) remained at 27 percent and the Greens, their favored coalition partner, stood at 15 percent, one point higher than the previous week.

The maverick Pirates shed one point to 3 percent and the socialist Left party was unchanged at 7 percent.

The poll of 1,850 people was conducted between February 21 and February 27.

(Reporting by Michelle Martin; Editing by Peter Graff)