A defiant Gordon Brown today declared that he was determined to stick to his principles over plans to extend the time terrorist suspects can be held without charge to 42 days.
The Prime Minister - who is facing a potential Commons defeat on the issue - said the "sophistication and complexity" of terrorist plots meant it was "inevitable" that the police at some point would need more than the existing 28 days.
He told a Downing Street news conference that the Government was putting in place the necessary safeguards to protect civil liberties.
Mr Brown said the security services were currently investigating 2,000 terrorist suspects involved in around 200 networks and 30 potential plots.
In the most recent case to come before the courts, he said police had to to examine 400 separate computers, 8,000 discs and 25,000 exhibits.
"The complexity and sophistication of investigations that now have to be conducted by the police, compared and contrasted with what was happening 10 years ago, make it inevitable that the police will need more time to conduct their investigations," he said.
Mr Brown insisted that he would not be deflected from the plan in the face of the threat that rebel Labour MPs could combine with the opposition to inflict a damaging defeat on the Government.
"I have tried to build consensus around our proposals but I am determined that we stick to our principles and that is that up to 42-days detention is and will be necessary in the future, but Parliament will make the final decision on the individual incident itself," he said.
Under the safeguards built into the legislation, he said that the police, the Director of Public Prosecutions and the Home Secretary would have to approve any order to invoke the additional power.
There would then have to be a vote by Parliament to implement the order.
Mr Brown said that there would also be new powers for the independent reviewer of terrorist legislation to examine individual cases.
Mr Brown said he believed the safeguards were "commensurate" with the needs of national security.
"I have looked at the civil liberties issues and we have made and are making proposals about how we can protect people from arbitrary treatment, but I believe that this is right in the interests of the national security of the country," he said.
His comments marked the start of a concerted push by ministers to win over Labour rebels before next week's crucial vote.
Home Secretary Jacqui Smith will appeal to Labour MPs tonight to fall in behind the anti-terror plans when she addresses the Parliamentary Labour Party (PLP).
With up to 50 Labour MPs ready to vote against the plans, Ms Smith has signalled there could be further concessions on when and how the new police powers will be used.
There were also efforts to woo the nine MPs of the Democratic Unionists to support the Government on this issue.
The vote comes at a difficult time for Mr Brown, who has seen his authority eroded after his embarrassing U-turn over the 10p tax row and a run of disastrous election results.Reuse content