Mr Justice Brooke granted Save our Railways leave to mount a full judicial review challenge and, if the pressure group wins its case, it may be too late to push through railway privatisation before the next election.
Save Our Railways is challenging rail franchise director Roger Salmon's decision in September to invite franchise tenders for four rail lines.
The judge's ruling prevents Mr Salmon from accepting any tenders for InterCity East Coast, Gatwick Express, Midland Main Line and Network SouthCentral until the outcome of a full hearing of the case on December 7 this year.
The judge said he was 'very concerned' by the delay in Save our Railways's bringing its challenge in respect of those three lines to court.
In the meantime Mr Salmon has undertaken to give the group 48 hours notice of any decision to accept tenders for those three franchises so as to give it the chance to make an emergency court application.
The judge said Save our Railways, which has the backing of 19 local authorities and the rail unions, had 'raised properly arguable points' that the Director had 'misconstrued' his instructions about the terms under which the franchises should be leased.
Nigel Pleming QC, for the group, told the court that Mr Salmon had been instructed that franchises should be offered with 'minimum service levels to be based on that being provided by British Rail.
'Throughout the rail privatisation debate, parliament and the public had been assured by the secretary of state for transport and the minister for public transport that at the time the franchises were offered there would be only minor changes to British Rail timetables,' Mr Pleming said.
Instead, minimum service levels imposed on franchisees by the government were so low that in some areas services would be slashed by over 50%.
Services provided by Great Western InterCity 'would be reduced by 50%'. Services to and from Newark 11%, Grantham 13%, Peterborugh 40% and Doncaster 52%, Mr Pleming gave to the court as examples.
'Nowhere in any of the minimum service levels is there a requirement to increase services,' he told the court.
The judge granted Save our Railways leave to seek judicial review of the director's decision to invite tenders for franchises for the four lines on September 14 this year. If the action succeeds, the group says that all seven franchises will have to be withdrawn and the entire process restarted which could delay rail privatisation to beyond the next general election.