Germany's Greens continued their upward trajectory on Sunday, with the ecologist party overtaking Chancellor Angela Merkel's conservatives in a new opinion poll just five months before a general election.

The centre-left opposition party rose to 28 percent in a poll for the Bild am Sonntag newspaper, gaining six percentage points on the previous week to record its best-ever score in the regular survey carried out by the Kantar institute.

Merkel's centre-right CDU/CSU alliance lost two points, coming in second at 27 percent support.

The centre-left Social Democrats (SPD), Merkel's junior coalition partners, fell to their worst score since August 2019 with 13 percent.

The Greens' surge in popularity comes after the party on Monday tapped co-chair Annalena Baerbock, 40, to become its first-ever chancellor candidate.

The nomination of mum-of-two Baerbock, seen as a centrist who advocates a greener economy and a tougher foreign policy stance on Russia and China, has been widely cheered.

She will also be the only woman in the race to succeed Merkel, who is bowing out after 16 years.

"Germans are in the mood for change," Bild wrote.

Another opinion poll last Tuesday for broadcasters RTL and NTV was the first to put the Greens in the lead, also giving them 28 percent support.

But political scientist Oskar Niedermayer at Berlin's Free University said the Green surge was mainly down to "media hype" that was unlikely to last.

The Greens are currently polling well among a wide range of voters, but this will change as soon as their election manifesto becomes "more concrete", he predicted. Some of the newly-won supporters "will drift away again", he told the Handelsblatt financial daily.

- Power struggle -

Merkel's ruling conservatives meanwhile have slumped in recent polls as voters punish them for perceived mistakes in the handling of the coronavirus pandemic, corruption scandals and party infighting.

Her bloc unveiled CDU leader and North Rhine-Westphalia state premier Armin Laschet as its chancellor candidate last week, following a bitter power struggle with CSU leader Markus Soeder. 

But Laschet, billed as the continuity candidate as Germany enters the post-Merkel era, is deeply unpopular among Germans.

Observers say there is a realistic chance the CDU/CSU will not re-emerge as Germany's largest political force after the September 26 vote.

Bavarian premier Soeder, who backed down in the fight for the chancellor candidacy despite strong grassroots support and higher approval ratings than Laschet, said at the weekend he was "not convinced" by Laschet's candidacy.

The CDU/CSU bloc "needs a fresh start", he told the Sueddeutsche newspaper.

Laschet sought to downplay the spat in an interview with the same newspaper, and said his top priority was to make sure the conservatives remain "the biggest power" in German politics.

He also lashed out at lawmaker Baerbock's lack of governing experience. "She talks. I take action," he said.