The prime minister is facing renewed pressure to drop plans to detain terror suspects for 42 days after a poll showed that fewer than one in three MPs thinks it will work.

Ipsos Mori questioned 116 MPs of all parties; 31% said it would have a positive impact on fighting Islamist terrorists. That compared with 44% who believed it would have either a negative impact or would make no difference.

The survey found that only 55% of the Labour MPs questioned believed that the law would have a positive impact; 20% said it would have a negative impact or make no difference at all.

The government is bracing itself for defeat when the proposal is debated in the Lords tomorrow. The rebellion is likely to be spearheaded by previously loyal supporters such as Lord Goldsmith, the former attorney-general, and Lord Falconer, the former lord chancellor. There will also be opposition from the Tories and Liberal Democrats who oppose the bill on civil liberties grounds.

The government is insisting that it has no plans to back down.

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