The joint committee on human rights said the plans, set out by justice secretary Ken Clarke, were "inherently unfair" and based on "vague predictions" and "spurious assertions".
In a letter to the National Security Council, seen by the Daily Mail, Clegg said he and his Liberal Democrat colleagues could not support the proposals in their current form.
The government proposals aim to make it possible for judges to take more evidence in secret in a small number of cases.
Baroness Berridge, who sits on the human rights committee in parliament, told BBC Radio 4's Today programme: "The proposals would bring about a fundamental change in the way in which courts would operate. To introduce such a fundamental change we need compelling evidence. We haven't seen any."
The committee said the plans were a "radical departure from long-standing traditions of open justice" and should only be allowed when there is "a real risk of harm to national security".