FILE PHOTO - German Chancellor Angela Merkel and Economy Minister Sigmar Gabriel attend a meeting at the lower house of parliament Bundestag on 2017 budget in Berlin, Germany, November 23, 2016. REUTERS/Fabrizio Bensch/File Photo

BERLIN (Reuters) - The chairman of Germany’s Social Democrats (SPD), Sigmar Gabriel, has decided to run against conservative Angela Merkel for the chancellorship in September’s federal election, the newspaper Bild reported on Tuesday.

The report backs up what senior party sources told Reuters last week, that there is no realistic alternative to Gabriel, vice chancellor and economy minister in Merkel’s right-left coalition of the last four years.

A spokesman for Gabriel said the party was sticking to its planned timetable. “The SPD will decide on its candidate for chancellor on Jan. 29,” he said. Senior party members are holding a closed-door meeting to discuss election strategy on Tuesday, but the question of who will be the party’s top candidate will not be discussed officially, Bild reported.

The paper said the SPD’s last chancellor, Gerhard Schroeder, had urged Gabriel to stand. “You must make clear that you really want it,” the newspaper cites Schroeder as saying, adding: “Otherwise I’m ready to do it again.”

Gabriel has long favored to stand against Merkel, but some party members had hoped that Martin Schulz, the former president of the European Parliament, would take on the role. That now looks unlikely, since he is tipped to become foreign minister, replacing Frank-Walter Steinmeier, whom ruling parties have agreed to elect as German president next month.

Gabriel, a 57-year-old former school teacher, trails Merkel in popularity ratings. He has a reputation for being unpredictable, but he is more popular with the SPD rank and file than his recent predecessors.

Although the SPD wants to win enough votes to form a coalition with the Greens and possibly the radical Left party, most analysts think another ‘grand coalition’ is the most likely outcome with Merkel clinching a fourth term.

An INSA poll in Bild on Tuesday showed Merkel’s conservatives on 32 percent, unchanged from last week, and the SPD also stable on 21 percent. The combined total for the SPD, Greens and Left was 41 percent compared with 53 percent for another grand coalition. It put the anti-immigrant Alternative for Germany (AfD) on 15 percent.