Jacqui Smith today defended the concessions she has made in an attempt to head off a backbench rebellion in a crucial Commons vote on the detention of terror suspects.

After speaking at a tense backbench meeting last night, the Home Secretary this morning made the rounds of the television and radio news studios in a determined campaign to promote her vision of terror law reform.

She is later due to table amendments to the Bill, and has promised to list in the legislation all the criteria which will now have to be met before it is possible to detain a terror suspect for 42 days.

The concerted Home Office drive to win over Labour waverers comes at a tense time for the Government, which will be further weakened if it loses next week's vote.

Gordon Brown has ruled out resigning if he loses, insisting yesterday that the vote would not be an issue of confidence. He said that the proposal would be put to Parliament “in the normal way”, meaning that it would not be a confidence vote - which would have been perilous, as the vote this morning still appeared on a knife-edge.

The Prime Minister has nonetheless put his authority on the line by refusing any concessions over the core of the 42-day detention plan.

Ministers and whips are agreed that Mr Brown is so weakened after recent setbacks that a Commons defeat would further erode his battered authority and throw the Government into turmoil. Referring to yesterday’s article by Mr Brown in The Times, in which he refused to water down the 42-day idea, a whip said: “He has decided to stand by his principles. That is all very well but we now have to save him from himself.”

As Ms Smith took her 42 days offensive to the airwaves this morning, she insisted that ministers were only proposing a “reserve power” to save having to rush legislation through Parliament at a later date.

If anti-terror officers actually wanted to use the power to hold a suspect for longer than the current 28-day limit, this would then have to be approved by a judge in individual cases.

The Home Secretary would have to go before Parliament and justify each use of the new powers, guaranteeing that all the criteria had been met, that the decision was urgent and compliant with human rights legislation.

MPs would have the power to vote to overturn their use within 30 days, or possibly even earlier.

“At the moment we are proposing it should be after 30 days, but certainly one of the things I was discussing with one of my colleagues last night was whether or not that should be brought forward," said Ms Smith, in an interview on the BBC Radio 4 Today programme.

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