The report by the council of experts on climate change, an independent body appointed by the government, adds to worries of climate policy backsliding under conservative Chancellor Friedrich Merz.

Europe's biggest economy has set a legally binding 2030 target to cut greenhouse gas emissions by 65 percent compared with 1990 levels, and a 2045 deadline to achieve climate neutrality.

The pace of reductions has slowed -- emissions were virtually unchanged last year -- but the environment ministry has insisted it is still possible to meet the 2030 goal.

However, the experts said the ministry's projections "underestimate the emissions expected up to 2030," and that it "assumes that the target will be missed".

Especially in the buildings and energy sector, emissions were likely to be higher than projected, council head Barbara Schlomann said.

Berlin's new climate action plan -- announced in March, and including measures such as electric car subsidies and a boost for wind power -- also does not go far enough and should be revised, she said.

The Greens party described the report as a "a stinging slap in the face" for Merz's coalition.

The government was "knowingly heading for a breach of the law that will come at a high cost to the people and the country," Greens lawmaker Julia Verlinden told the Rheinische Post newspaper.

But an environment ministry spokesman signalled the government still believed it was on track to achieve its ambitions, adding that the experts were basing their assessment on "more conservative assumptions" in areas like wind power.

Environmental Action Germany (DUH) -- which successfully sued the previous government over a climate plan that courts determined was inadequate -- has already vowed to challenge the plan in court.

The group said Monday's report had given them extra impetus to push on with the challenge, saying the climate programme must be improved "as quickly as possible".

The government's plans have "nothing to do with reality," charged DUH federal managing director Juergen Resch.

Merz's government, arguing that onerous regulations are burdening Germany's struggling economy, has looked to scale back climate policies in some areas.

He has backed a loosening of EU-wide car emission rules while his energy minister has pushed plans to build new gas-fired power stations.

But government spokesman Stefan Kornelius insisted Monday that "climate policy is no less important to this government than it was to the previous one".