BERLIN (Reuters) - Three left-leaning German parties would now win enough combined support to oust Chancellor Angela Merkel in September's election, according to an opinion poll by the INSA institute to be published in Bild newspaper on Tuesday.

The newspaper said in advance of the report that it was the first time "in a long time" that a left-of-center coalition would have enough support to knock the Christian Democrats out of office.

The three parties had enough votes to form a coalition after the last election in 2013 but opted not to join forces because of lingering SPD opposition to alliances with the hard-left Linke party. Support for all three parties fell after that and was long short of a majority before the INSA opinion poll.

The poll found that the Social Democrats would win 31 percent to 30 percent for the conservatives, unchanged in the last week. The Linke party would win an unchanged 10 percent and the pro-environment Greens were also steady, winning seven percent for a total of 48 percent for all three leftist parties.

That would leave just 47 percent for the three other parties likely to clear the 5 percent threshold and win seats in the next parliament, the poll of 2,028 voters found.

The CDU and their Bavarian sister party would win 30 percent, the Free Democrats (FDP) 5 percent and other smaller parties below the 5 percent threshold, according to the poll.

(Reporting by Erik Kirschbaum; editing by Grant McCool)