Germany's phase-out of nuclear energy has triggered over 20 lawsuits by big power companies who have demanded billions of euros in damages, said a government paper released Tuesday.

Berlin after Japan's 2011 Fukushima nuclear disaster moved to immediately shutter the country's eight oldest reactors and close all others by 2022 while boosting renewable energy such as wind, solar and biomass.

Three large electricity companies -- EON, RWE and Vattenfall -- have responded with a spate of court challenges, which the environment ministry has listed for the first time in response to a request by the Greens party.

In total, 14 complaints have been filed against the national government, including nine cases before the top Constitutional Court, while seven cases have been filed against state governments.

The likely final bill if the lawsuits succeed is unknown but would probably run into the billions, according to the paper requested by Greens party lawmaker Silvia Kotting-Uhl and made available to AFP Tuesday.

The biggest known claim came from Swedish company Vattenfall, which has demanded 4.7 billion euros ($5.8 billion) compensation from the German federal government.

Vattenfall launched its claim in 2012 before a Washington-based tribunal, the International Centre for Settlement of Investment Disputes.

RWE has meanwhile demanded 235 million euros in damages in a German court, and EON is suing for 386 million euros, said the ministry.

The only one among the "big four" power companies in Germany that has not launched a major suit is EnBW, which is now part owned by Baden-Wuerttemberg, a southwestern state ruled by the Greens.

EnBW has however joined lawsuits against a nuclear fuel rod tax, which would also be expected to run into the billions.

Other lawsuits, some over nuclear waste storage on power plant sites, have been launched against Bavaria, Hesse, Lower Saxony, Rhineland-Palatinate and Schleswig-Holstein states, the ministry said.

In other legal actions, companies are demanding access to official documents.