Charles Kennedy, the Liberal Democrat leader, will today attempt to fend off criticism of weak leadership with a strong attack on the government's proposed anti-terror measures and its handling of postwar Iraq.

The Lib Dems are threatening to withdraw their support for a package of new anti-terror laws if the government insists on giving police the power to detain terror suspects without charge for up to three months. Mr Kennedy will tell his party's conference today that he will not accept a "blanket extension of custody powers" - from 14 to 90 days - "a prison sentence by any other name".