BERLIN  Mon Apr 20, 2015 8:06pm BST

Germany's environment minister on Monday hit back at opposition from leading members of Angela Merkel's conservatives to a proposed coal levy, saying it risked jeopardizing the chancellor's credibility ahead of the G7 summit in June.

Germany has made climate one of the key issues of its G7 presidency and hopes to use the summit in Ellmau in Bavaria to encourage others to pledge tough goals to cut greenhouse gases ahead of a U.N. climate summit in Paris in December.

But opposition to plans to force operators of coal plants to curb production at their oldest and most polluting power stations could put Germany's climate goals at risk, Barbara Hendricks told a press briefing on Monday.

"It will be very difficult for the chancellor to travel to Ellmau if she can't implement her own goals," said Hendricks, a member of the Social Democrats (SPD), who are the junior partner in Merkel's coalition government.

Concerned it was on track to fall short of its target to reduce CO2 emissions by 40 percent by 2020 from 1990, the German cabinet approved a climate package last December.

This included plans to oblige coal operators to slash their emissions by at least 22 million tonnes by 2020, equivalent to shutting about eight coal plants.

But proposals by Economy Minister Sigmar Gabriel to force the operators of coal plants to curb production has sparked a backlash from energy groups and German states worried about a loss of jobs.

On Monday, Volker Kauder, parliamentary leader of Merkel's conservatives, told a German newspaper that Gabriel's proposals risked wiping out the coal industry and could not be implemented. Fellow senior conservative Armin Laschet has also rejected the plans.

Hendricks said that by flatly rejecting the proposals without coming up with alternatives, they were "stabbing the chancellor in the back".

"It's about adhering to goals to which Germany is internationally bound," she said.

Energy groups and trade unions also oppose the proposals. Coal union IG BCE plans to stage a demonstration in Berlin on Saturday against the coal levy, which it says will put 100,000 jobs at risk.