Government plans to hold more inquests and court cases behind closed doors appeared to be in tatters after Deputy Prime Minister Nick Clegg told Cabinet colleagues he could not support the proposals in their current form.
The intervention by Mr Clegg came as MPs and peers on the cross-party Joint Committee on Human Rights savaged Justice Secretary Ken Clarke's "inherently unfair" plans.
Mr Clegg's concerns were set out in a letter to the Government's powerful National Security Council in which he warned that his Liberal Democrat colleagues would not be able to back the proposals without major changes.
The plans are aimed at finding a way of managing sensitive evidence from the security services, but Mr Clegg said their concerns "cannot be allowed to ride roughshod over the principles of open justice".
He said the powers to take evidence in secret should not apply to inquests and that judges rather than ministers should decide when the measures are used in a small number of civil cases expected to be affected, the Daily Mail reported.
The Joint Committee on Human Rights heaped further pressure on Mr Clarke to perform a U-turn in a damning report which said the controversial and wide-ranging proposals were based on "vague predictions" and "spurious assertions" about catastrophic consequences.
In reality, the plans are a "radical departure from long-standing traditions of open justice" which should only ever be used when publicly disclosing material would carry "a real risk of harm to national security", the committee said.
Dr Hywel Francis, the committee's chairman, added it was "troubling" that the justice and security green paper "was not as clear as it should have been on the scope of its proposals or the narrowness of the justification for changing the law".
The report strongly criticised Mr Clarke's green paper, saying his view that the plans were only intended to be used in a small number of cases "is clearly a change of position as there is no doubt that the proposals in the green paper are very broad in scope".
Responding to the committee's report, Mr Clarke said the Government's proposals were "a common sense solution to a genuine problem in a very small number of cases". He said: "They will ensure that the Government is properly held to account when individuals challenge its actions in civil cases only, without revealing information which would compromise public safety."