Government plans for "secret courts" to hear evidence relating to national security in private threaten to open a fresh Coalition rift.

Senior Liberal Democrats will raise fears over the proposals at their party conference next month as they attempt to distance themselves from the Tories over civil liberties.

The criticism is being privately supported by Liberal Democrat ministers who are anxious the plans could undermine fundamental principles of open justice.

Initial proposals by Kenneth Clarke, the Justice Secretary, for a wide extension of secret courts in civil cases have been scaled back in the face of protests from lawyers. Inquests have been dropped from the original provisions and judges, rather than ministers, will now have the power to hold sessions behind closed doors.

Mr Clarke insists the move is essential to allow sensitive material to be used to protect the Government from vexatious civil cases. But critics say the revised plans still put the intelligence services above the law and judges would be required to comply with ministers' requests for secret sessions.

A strongly-worded motion to the Liberal Democrat conference from activists calls for the plans to be dropped by the Coalition altogether.

The MP Tom Brake, co-chair of the party's backbench home affairs committee, said concerns remained that judges would be genuinely free of political interference.