Phone tap evidence

The new Metropolitan Police commissioner, Sir Ian Blair, is in favour of allowing phone tap evidence to be admissible in courts and, contrary to expectations, the head of "Liberty" is also in favour!! This does seem reasonable in the face of the supposed "terrorist threat". Why then is Charles "Lock them up without a trial" Clarke against the idea? Is it because the suspects will already be banned from using phones so therefore there will be no phone to tap.