Secret court plans are mainly aimed at reducing "reputation and political costs," according to a Civil Service report on the Justice and Security Bill, which is to be debated tomorrow in the Commons.
Ken Clarke, who is attempting to drive the Bill through the house, has claimed that the plans will prevent the government being forced to pay out millions in civil cases and protect national security.
But the government's own impact assessment says that the plans will actually cost millions and are aimed mainly at reducing "reputation and political costs to the UK."
The document, which has been sneaked out without fanfare, also finds that the Bill "may lead to negative impacts on UK nationals and residents and businesses."
Legal action charity Reprieve said the assessment findings reveal that the main motive for secret courts is to spare the embarrassment of ministers and their officials.
"It is appalling that the government is undermining centuries-old British legal freedoms in order to avoid 'political costs' - or in other words, embarrassing headlines," executive director Clare Algar said.
"Almost nothing that has been said by Ken Clarke in defence of these dangerous plans can be believed." She urged MPs to vote against them next week.
Meanwhile it emerged today that the Special Advocates, who it is proposed would act for clients under the proposals, have attacked the plans as "inherently unfair and contrary to the common law tradition."
In a note submitted to Parliament's joint committee on human rights they said the government had submitted no evidence in support of the plans, adding that "in our view, none exists."
The Justice and Security Bill sets out the plans for a roll-out of secret courts or closed material procedures (CMPs).
The use of CMPs would mean that, in cases brought against the government, ministers would be able to exclude their opponents, the press and the public from the court room.
The Special Advocates warn that under CMPs "it will be possible to have proceedings in which the court's decision is based entirely on evidence about which one of the parties has been told nothing at all."
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