The environmental group, joined by Lancashire County Council, is claiming that John Gummer, Secretary of State for the Environment, and Gillian Shephard, Minister of Agriculture, were wrong to grant British Nuclear Fuels (BNFL) authorisations for radioactive emissions from Thorp without first holding a public inquiry.
If Mr Justice Potts finds against the Government, it would delay the start of full-scale operations for months and raise the prospect of the pounds 2.8bn plant never opening.
Thorp, at Sellafield on the Cumbrian coast, was completed more than a year ago and BNFL says the delays are costing nearly pounds 2m a week.
About 2,000 people work at the plant, which reprocesses spent fuel from nuclear reactors into uranium, plutonium and radioactive wastes.
BNFL has just moved the first three tonnes of spent fuel into the plant and wants to fragment it within the next fortnight. That will be the point of no return - from then on, parts of the plant will be highly radioactive and expensive to decommission.
Andrew Collins QC, counsel for Greenpeace, told the court: 'The process leading to the authorisations was flawed and there ought to have been a public inquiry before the relevant decisions were taken.'
He alleged that the ministers had failed to consult the public properly, failed to provide necessary information during the consultation process, and 'misunderstood what they should have taken into account'.
There were two rounds of consultation last year. Mr Gummer said he was not legally obliged to carry out the second round - on the broader question of justification for the plant - but had wanted to.
Mr Collins said it was not a 'genuine consultation process'. He claimed the Government had only embarked on it 'to protect its back' from judicial review.
He said BNFL should have made several key documents publicly available, including contracts with foreign clients who will send nuclear waste to Sellafield and a report by Touche Ross, the accountants, which is said to outline the economic justification for the plant. Even Mr Gummer had not seen the Touche Ross report, he added.
The scale of the annual radioactive emissions from Thorp meant that the authorisations involved were 'the biggest there have ever been, or are ever likely to be', Mr Collins said.
Lancashire has decided to join Greenpeace's fight because it borders Cumbria and believes a small number of people within the county could receive annual doses of radioactivity, slightly above the Government's precautionary limit, due to the plant's operations.
The costs of the hearing, which will last at least five days, are expected to run to several hundred thousand pounds. Greenpeace has raised more than pounds 200,000 through an appeal and has also secured the backing of Sir James Goldsmith, the Anglo-French tycoon who has environmental sympathies.
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