Greenpeace has asked a High Court judge to stop the Government providing a £650 million bailout to British Energy.
The environmental pressure group is seeking a judicial review of the decision, which it argues breaches EU competition law and puts other green generators at a disadvantage.
"We think we've got a very strong case, although that's no guarantee that we'll actually win it,
said nuclear campaigner Rick Le Coyte.
The case is the sixth review of a ministerial decision requested by Greenpeace in the UK. Coyte admitted that cases were expensive but always worth doing because they discouraged agencies from breaking the law.
"This money damages other generators, such as wind energy, because it skews the market for electricity,
said Le Coyte.