Senior civil servants say privately that publication of the report, part of which has been seen by the Independent on Sunday, has been held up to avoid embarrassing the Government, which will almost certainly give the go-ahead for the pounds 2.8bn plant this week.
Officials also admitted last week that key government departments failed to ask British Nuclear Fuels (BNFL) for crucial documents about its plant throughout a year of 'public consultation' on its future.
A Conservative MP who was permanent private secretary to one of the Cabinet ministers making the decision is now demanding a public inquiry.
The suppressed report, on ministers' desks since August, gives unprecedented backing to the 'dry storage' of nuclear fuel, the alternative to reprocessing which Thorp's opponents have advocated for nearly two decades.
The report is embarrassing to the Government on two counts. It contradicts part of the case that ministers have put for Thorp. And it is the result of a public inquiry - which environmentalists are demanding before a final decision. The little-noticed inquiry - which opened in Dunbar, Lothian, more than a year ago - was held into an application by Scottish Nuclear, one of Thorp's biggest customers, to build dry storage for used fuel at its Torness station. Instead of being sent to Sellafield to be reprocessed into plutonium, uranium and nuclear waste, it would be stored.
Environmentalists have been arguing for dry storage since the mid-1970s because it causes less pollution and avoids the risk of nuclear proliferation.
While publicly supporting Thorp, Scottish Nuclear in fact plans to stop using the plant in 1996, and to store all its spent fuel instead. Robin Jeffrey, its managing director, says this would save his firm pounds 45m a year and 'bring down the cost of nuclear electricity in the UK'.
The inspector, R M Hickman, submitted his report to Ian Lang, the Scottish Secretary, in August. Scottish Nuclear says that it is 'confident' that the report comes down in favour of dry storage, and a senior civil servant said last week it was being suppressed 'because it would be very embarrassing if it came out at this stage'.
In his 'findings of fact', seen by the Independent on Sunday, the inspector refutes two key arguments in the case for Thorp - that reprocessing makes it easier to dispose of nuclear waste, and that even stored fuel would be reprocessed at some stage.
Dr Patrick Green, Nuclear Campaigner for Friends of the Earth, said last week: 'The Government's suppression of the report has kept from the public vital information which calls the whole basis of Thorp into question. It shows that its public consultation is a sham.'
Tory MP Julian Brazier, who was parliamentary private secretary to Gillian Shephard, is calling for a public inquiry after reading in the Independent on Sunday that a Pentagon report concluded that Thorp would greatly increase the risk of nuclear proliferation.
BNFL says it abides by all national and international requirements for plutonium shipment.