Chancellor Angela Merkel's successor as leader of Germany's ruling Christian Democrats (CDU) is under growing pressure after the party was forced into second place by the Greens in a series of opinion polls.

Three polls published in three days showed the Greens pulling ahead of the CDU, making them the country's most popular party for the first time since they were founded.

The latest, a Forsa GmbH poll published yesterday, put the Greens on 27 per cent, three points ahead of the CDU - a historical low for the traditional governing party of Germany.

The Greens have enjoyed a surge in popularity in recent months amid growing concern about climate change, biodiversity and plastic waste, and overtook the Social Democrats, their left-wing rivals, earlier this year.

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Critics within the CDU have blamed Annegret Kramp-Karrenbauer, who took over from Merkel as party leader in December, for a string of tactical errors.

Particular criticism has been levelled at the party's chaotic performance during the campaign for the European Elections last month, when a YouTube video attacking the party's record on climate change and income inequality went viral.

The CDU were accused of panicking after they shot their own video in response to the attack, only to axe it before publication.

Talk is already turning to whether Kramp-Karrenbauer is the best person to lead the CDU into the next election. The 56-year-old, who won a leadership election after Merkel decided to stand down as head of the party, came out fighting today, warning that the Greens were too ideologically close to Die Linke, a party of the radical Left.

"People who dream of a new Green-led government need to know that they could wake up with Die Linke in power," she told Bild am Sonntag.

Her comments came after the Greens chose to enter coalition talks with Die Linke, rather than the CDU, following elections in the state of Bremen last month.

"Bremen shows that, when in doubt, the Greens pick the left-wing over the politics of the middle ground," Kramp-Karrenbauer said.