Proposals to let police detain terrorism suspects for up to six weeks without charge are wrong in principle and unworkable in practice, former intelligence chief Eliza Manningham-Buller said on Tuesday.

The cross-bench peer was making her maiden speech in the House of Lords, which was debating the counter-terrorism bill for the first time.

"I've weighed up the balance between the right to life, the most important civil liberty, the fact that there's no such thing as complete security, and the importance of our hard-won civil liberties," former MI5 chief Manningham-Buller said.

"And therefore on a matter of principle I cannot support the 42 days pre-charge detention in this bill.I don't see on a practical basis as well as a principled one that these proposals are (in) any way workable."

She argued that issues of national security should be placed above party politics.

The government narrowly won a House of Commons vote last month on the 42-day detention period, which it says may be needed by police in the event of a "very grave and exceptional terrorist threat".

Critics say the bill is draconian and an affront to civil liberties. The House of Lords cannot kill the legislation, but can embarrass the government by delaying its passage.