Mr Justice Mitting yesterday upheld a legal challenge from Friends of the Earth and two solar energy companies - HomeSun and Solarcentury - ruling that it would be illegal for the cuts to have an "effective date" on the 12 December, two weeks before the consultation officially ended.
The ruling quashed a proposal for any installation completed between 12 December and 1 April 2012 to receive the current 43p/kWh level of feed-in tariff payment until 1 April, before switching to the lower proposed level of 21p/kWh.
However, it now remains unclear whether installations completed in that period will be eligible for the current higher feed-in tariff rate for the full 25-year payment period because the government is planning to request an appeal.
Mitting yesterday rejected an immediate application from the Department of Energy and Climate Change to appeal and downplayed its chances of success at a further request.
However, Edmund Robb of Prospect Law, who acted for Solarcentury, told BusinessGreen that the 12 December deadline could be reinstated if the government's appeal is successful.
"This challenge has found that the 12 December deadline is unlawful and anyone who registered for feed-in tariffs between now and 31 March is entitled to the current rate for the full 25 years," he said.
However, if DECC is successful in any appeal the 12 December deadline could be reinstated and those people who registered after 12 December will only be entitled to the current rate until the end of March.