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BERLIN (Reuters) - German Chancellor Angela Merkel’s conservatives expanded their support to 40 percent - the highest level since September 2015 - in a poll conducted by the Forsa institute for Germany’s Stern magazine and broadcaster RTL.

The rival Social Democrats (SPD) were unchanged at 23 percent, while the pro-environment Greens gained 1 percentage point to 9 percent and the anti-immigrant Alternative for Germany (AfD) party was unchanged at 7 percent, Forsa said.

The far-left Linke party lost 1 point to 9 percent, and the pro-business Free Democrats dropped 1 percentage point to 7 percent, in the poll of 2,502 eligible voters that was conducted on June 19-23.

It showed Merkel’s Christian Democrats at their highest support level since September 2015, shortly after Merkel’s decision to open the doors to more than 1 million migrants, mostly from the Middle East and Afghanistan.

Forsa said 52 percent of those polled would back Merkel for a fourth term if the chancellor were elected directly, a drop of 1 percentage point, compared with an unchanged 22 percent who would back SPD leader Martin Schulz, the former president of the European Parliament.

Under Germany’s complex electoral system, voters cast ballots for parties, not for chancellor. The chancellor is usually the top candidate from the party that wins the most votes, and must be approved by the newly elected members of parliament in a secret ballot.

Only 9 percent of eligible voters trusted the SPD to solve Germany’s problems, compared with 40 percent for Merkel’s conservatives, its highest competency rating since October 2013, shortly after the previous national election.

The poll has a margin of error of plus or minus 2.5 percentage points.