BERLIN, Aug 30 (Reuters) - Germany"s centre-left Social Democrats (SPD) extended their lead over Chancellor Angela Merkel"s conservatives in the latest poll published on Monday, increasing the chances of a left-leaning coalition government following a general election next month. The Insa poll for Bild TV showed support for the SPD and their chancellor candidate Olaf Scholz jump by 2 percentage points to 25%, the highest reading in the survey in four years. Merkel"s conservatives and their top candidate Armin Laschet lost 3 points to reach a record low at 20%. The ecologist Greens stood at 16.5%, the business-friendly FDP at 13.5%, the far-right AfD at 11% and far-left Die Linke at 7%. Insa head Hermann Binkert said Laschet could currently only become chancellor with a three-way "Jamaica coalition" consisting of the black conservatives, the Greens and the yellow FDP. "Olaf Scholz would have four options in which to lead the government," Binkert added. This included a so-called "traffic light" coalition with the Greens and the FDP and a more left-leaning coalition with the Greens and Die Linke. Another possibility would be an SPD-led coalition government supported by the Greens and conservatives, or an SPD-led coalition supported by the conservatives and the FPD. The conservatives rallied behind Laschet, promising to go on the offensive after he struggled to revive their campaign in a televised debate with rivals. Laschet, leader of Merkel"s Christian Democrats (CDU), was judged to have lost the heated televised debate with his two main rivals on Sunday, a snap poll suggested, as surveys showed his party falling behind the SPD. Laschet has been under fire since he was caught on camera laughing during a visit last month to a flood-stricken town, but senior party officials praised his combative style in Sunday"s debate even if it did not immediately appear to win over voters. "We have to fight for the next four weeks," Jens Spahn, a Laschet ally and CDU health minister, told Bild TV, rejecting suggestions that the conservative alliance should drop Laschet in favour of Bavarian conservative Markus Soeder. "You don"t change the coach during the game," Spahn said. The CDU"s slide marks a remarkable fall for the party after 16 years in office and four straight national election victories under Merkel, who plans to step down after the election. Soeder, who earlier this year ran against Laschet to be the conservative chancellor candidate, said his erstwhile rival had laid a good foundation for the next three-way debate in two weeks. "From my point of view, this was exactly what we had hoped for to give us new momentum," Soeder told reporters in Munich. "It also motivates our own election campaigners." Laschet on Monday brushed off the post-debate snap poll, which showed that of voters surveyed by pollster Forsa, 36% believed SPD candidate Olaf Scholz won, ahead of 30% for Greens candidate Annalena Baerbock and 25% for Laschet. Scholz is the most popular of the candidates in the polls. Despite the SPD"s lead, they would still need to team up with two other parties to govern.