According to a report in news magazine Der Spiegel, the energy companies have hired top law firms to prepare their case, including Linklaters, Freshfields Bruckhaus Deringer, Clifford Chance and Gleiss Lutz.
A document drawn up by Gleiss Lutz says the energy companies own the surplus electricity generated by their nuclear power stations, which is therefore protected by property law in the German constitution.
The lawyers say the government has failed to supply "stringent reasons" for their new phaseout plan, drawn up over the past three months, and that the energy companies are therefore entitled to compensation – amounting to several billion euros.
If a deal is not struck with the government, the Swedish firm Vattenfall is even threatening to take a case to international courts over their nuclear reactor Krümmel, which was shut down permanently in mid-March.
The companies are also planning to challenge the government's tax on nuclear fuel.
Horst Seehofer, head of the Christian Social Union, dismissed the legal challenges as hopeless. "We are not going to make a deal. Our actions are legally flawless and politically independent," he told the Financial Times Deutschland.
Chancellor Angela Merkel's cabinet signed a package of bills this month that foresee Europe's biggest economy being nuclear-free by 2022 - earlier than previously envisaged.
Germany's nine reactors currently on line are due to be turned off between 2015 and 2022.
The seven oldest reactors were taken off-line after Japan's massive March 11 earthquake and a tsunami knocked out cooling systems at the Fukushima Daiichi plant, causing reactors to overheat and radiation to be released.