A German court on Tuesday ruled that energy suppliers can claim compensation over the country's nuclear power phase-out following the 2011 Fukushima disaster, dealing a blow to one of Chancellor Angela Merkel's flagship policies.

Judges did not agree with power plant operators that the shutdown ordered by lawmakers in 2011 amounted to an "expropriation" of their assets, but said the government should agree a deal to compensate the firms by June 2018.

"It was permissible for lawmakers to take the accident in Fukushima as a prompt to speed up exiting nuclear energy to protect the health of people and the environment," senior judge Ferdinand Kirchhof told the court in Karlsruhe.

Although the phase-out decision itself was legal, the court found, the firms have a right to "appropriate" compensation -- which is not provided for in the law as it stands.

The judges did not specify how much the compensation should be, but media reports said the plaintiffs -- German electricity giants EON and RWE and Sweden's Vattenfall -- had sought some 20 billion euros ($21 billion) in damages.

Merkel's government decided after Japan's 2011 Fukushima reactor meltdowns to halt operations of Germany's eight oldest nuclear plants and to shutter the other nine by 2022.

The move marked a sharp reversal for Merkel, who had earlier overturned a phase-out ordered by a previous government in 2002.

"With today's judgement from the highest German court, we have clarity in a fundamental legal question for our business and its owners," RWE Power chief executive Matthias Hartung said in a statement.

EON's CEO Johannes Teyssen told the court in March that the government's decision had penalised many small shareholders who had put their savings and pensions into the company, the largest energy provider in Germany.

In a statement Tuesday, EON said it had invested "several hundred million euros" into extending the life of its nuclear plants before the 2011 decision.

The company said it would now study the verdict and was prepared "to enter into constructive talks" with the government.

Shares in both EON and RWE climbed in Frankfurt trading on news of the decision.

EON gained around 2.75 percent by 1410 GMT, while RWE added 1.5 percent against a market up 0.33 percent on the day.

- 'More expensive for taxpayers' -

The firms have complained that the expensive exit from nuclear comes as they are already battling low wholesale electricity prices and competition from heavily subsidised renewables as part of Germany's shift to clean energy such as wind, solar and biomass.

Critics counter that the big energy companies benefitted from massive state subsidies when the nuclear plants first went into operation.

Environment Minister Barbara Hendricks welcomed the court's confirmation that the nuclear phase-out was legal.

She added in a statement that since the judges had only partially upheld the firms' complaints, the "demands for billions are now off the table".

The verdict could however have an impact on parallel negotiations between the government and nuclear plant operators on managing the country's atomic waste disposal.

Under a draft law approved in October, the firms Vattenfall, EON, RWE and EnBW would have to contribute 23.5 billion euros to a state fund for the storage of nuclear waste by 2022.

But the agreement is yet to be finalised, and the plaintiffs may well use the favourable verdict as leverage in the talks.

Energy policy expert Claudia Kemfert of the DIW think-tank said with Tuesday's ruling, Germany's nuclear power exit was "becoming even more expensive than thought for the general public".

Safely decommissioning all the plants and storing their radioactive parts and waste could cost up to 50 billion euros, experts estimate.

Eight nuclear power stations remain in operation in Germany today.

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