MPs of all parties condemned moves to double the time terror suspects can be held without charge amid fears that the number of people jailed for terrorist offences could grow twelvefold in the next decade.
Jacqui Smith, the Home Secretary, vowed to press ahead with extending the 28-day limit on the time terror suspects can be questioned as ministers braced themselves for parliamentary warfare over the proposals.
The future scale of terrorism convictions was revealed in internal Whitehall documents which estimate that the number of extremists in jail could leap from the current 131 to 1,600 by 2016.
Ms Smith insisted: "There is a clear trend of growing complexity, growing numbers of people involved, of international links, that make it at least highly possible that at some point, longer than 28 days may be necessary for police and other investigators to investigate, interview, sift the evidence, make the international links to be able to charge somebody."
Ms Smith refused to specify a new limit, but ministers have said they favour increasing the maximum detention period to 56 days.
The Government also sprang a surprise by announcing plans to allow police to question all criminal suspects – not just those believed to be terrorists – after they have been charged.
David Davis, the shadow Home Secretary, said the 56-day proposal represented "a permanent, undeclared state of emergency". He said: "Extending detention without trial will, like ID cards and control orders, undermine our freedoms, but it will not make us safer. In fact, it risks making the threat worse."
Nick Clegg, the Liberal Democrat home affairs spokesman, branded extending detention without trial "divisive, wrong in practice and wrong in principle". He said: "Rushing forward without compelling reason to do so risks alienating opinion in exactly those communities we need on our side. Of course extremists, preachers of hate, will seek to radicalise youngsters whatever we do. But surely we have learnt by now that breathless talk about the war on terrorism or sloppy anti-terror legislation gives them needless additional ammunition to sustain their twisted and misguided grievances."
Bob Marshall-Andrews, the Labour QC and trenchant critic of the Government, said ministers had failed to give "a single example, not one" to justify extending the 28-day limit.
Patrick Mercer, the former Tory homeland security spokesman and an adviser to the Government on terrorism, warned: "Our enemies will label what we are doing simply as internment."
Mohammed Abdul Kahar, who was shot in the shoulder during the botched Forest Gate anti-terror raid last year, urged MPs not to back the Government's plan.
"If you give them [the police] more time, they do everything slower," he told the Home Affairs Select Committee yesterday.
"It is just prolonging the time. It's more modernised torture in a way. It is a form of torturing people."
The committee is taking the unprecedented step of calling Jonathan Evans, the head of MI5, to give evidence on the merits of exceeding the 28-day limit. On Monday Mr Evans revealed that al-Qa'ida was targeting teenagers as he warned that 2,000 people were involved in terrorist-related activities in Britain.