Power to detain suspects for up to 42 days would be used only to deal with a “grave, exceptional terrorist threat” to the United Kingdom, ministers said last night.

As they tried to head off defeat in next week's voting, ministers emphasised that the controversial plan would be limited to cases, similar to the London bombings of 2005, that cause or threaten a serious loss of life. It could also include attacks or plots abroad if they have implications for British security, vital energy supplies or transport links.

The legislation would be used either to thwart terrorist attacks or to deal with the aftermath and would be limited to investigations involving the most serious offences, including murder and conspiracy to cause explosions, which carry a life term.

The 42-day power could not be used where prosecutors were investigating a lesser offence, such as weapons training or the financing of terrorism.

Jacqui Smith, the Home Secretary, confirmed that the time in which police could use such power would be halved to 30 days and MPs would have to approve the power being invoked within a week rather than a month. The Home Secretary would be able to grant the powers only if the police and the Director of Public Prosecutions (DPP) said that they needed longer than 28 days to gather evidence.

Sir Ken Macdonald, the present DPP, is among senior legal figures who have dismissed the need for an extension of the present limit.

David Davis, the Shadow Home Secretary, said: “Yet again, Gordon Brown has buried an issue of high principle in a blizzard of fine print.

“These amendments are a deception. The Home Secretary's power to extend the period of detention without charge to 42 days remains as wide as before — and is certainly not confined to a state of emergency.

“They are a politically driven attempt to deceive Parliament. A Labour minister promised a series of 'Mickey Mouse' amendments to buy-off Labour rebels. The Government has delivered.”

Shami Chakrabarti, director of the pressure group Liberty, said that even with the proposed changes, the power could still become “routine, triggered for operational convenience in individual cases”.

Chris Huhne, the Liberal Democrat home affairs spokesman, said that the change was “a con”. “Holding someone for 42 days without [them] knowing the charges against them is nearly four times as long as the maximum 12-day period in any comparable English-speaking country.”

In a speech at the Smith Institute in London last night, the Home Secretary said: “Faced with the need to intervene early to protect the public, faced with complex international plots, more time may be needed to gather admissible evidence and get to the bottom of a plot.”

VERDICT IS STILL ON HOLD

The Government first tried to extend the period that terrorism suspects could be detained without charge to 90 days, but Labour MPs threw out the plans - inflicting Tony Blair's first Commons defeat in 2005.

Gordon Brown sought initially to appease rebels by reducing the period to 42 days and offering MPs a vote to approve the detention powers within 30 days when he returned to the issue earlier this year. That was reduced to seven yesterday. A

dditionally, the Home Secretary must now declare that there are “grave and exceptional circumstances” for using the power, although there is no legal requirement for the Government to prove that this is the case.

Ministers have also halved from 60 days to 30 the period that the powers can be used after parliamentary authorisation. However, the Government drew back from handing judges the right to quash the detention powers.

Francis Elliott

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