MINISTERS yesterday admitted that they want to hold terror suspects for up to 56 days without charge as the government prepared for another battle to extend detention limits.
Jacqui Smith, the Home Secretary, said that police would need more than 28 days to investigate as terror cases became more complex.
But the government's case for an extension of the detention limit was weakened when Ms Smith admitted that she could not name an example of when more than 28 days was needed.
The Conservatives yesterday promised to fight attempts to increase the limit. David Davis, the shadow home secretary, said there was no evidence that an extension beyond 28 days was needed. He argued that extending the limit would undermine hard-won freedoms without increasing security and risked acting as a recruiting sergeant for terrorism.
Mr Davis said that the debate was a rerun of the government's failed attempts to impose a 90-day limit in November 2005.
During a parliamentary debate, he also stressed that in a worst-case scenario - a state of emergency - the government already has powers to detain without charge for 58 days.
Earlier, Ms Smith told BBC Radio 4's Today programme: "There is a strong chance in the future we will need more than 28 days." She added that the terrorist threat was "serious, sustained and growing".
However, when questioned on whether there had been any cases so far where the 28-day limit had been insufficient, she admitted: "There has not."
Ms Smith was unable to say how long suspects should be held, but Downing Street clarified the government's position by saying ministers wanted a maximum of up to 56 days.
The contentious issue was put back on the agenda after a counter-terrorism bill was announced in Tuesday's Queen's Speech. The bill also proposes post-charge questioning specifically for interviewing suspected terrorists.
In a surprise announcement, Ms Smith also signalled a separate review of police powers to allow suspects of crimes other than terrorism to be questioned after being charged.
Q & A: COMPLEX INVESTIGATIONS
WHY does it take so much time to investigate terror cases?
The foiled airline bomb plot to blow up ten jumbo jets across the Atlantic involved investigating 200 mobile phones, 400 computers and 70 homes.
Inquiries were held across three continents and the scale of the investigation was used by Gordon Brown as an example of why the detention limit needed to be extended.
The sheer volume of information takes time to process by police and time for the Crown Prosecution Service to decide on charges.
Terrorists have also become more technologically savvy. Much of the evidence stored in computers is encrypted and takes weeks to decode by computer forensics experts. If suspects had to be released before the data was interpreted, it could endanger the public.
Police also want to intervene in the early stages of a plot to stop the would-be terrorists from carrying out an attack. To do this, and find enough evidence in the early stages of a plot to charge them, takes time.
The terror network stretches so far and wide that it is difficult to collect information on individuals, particularly if they are using multiple identities and travel across continents.
Foreign agencies are often tapped up for information and suspects are interviewed abroad through interpreters, which can slow down the process.
Any investigation involving hazardous substances such as chemicals has to be carried out with precision.
Why is it back on the agenda now?
The Queen's Speech included a Counter-Terrorism Bill, which will be published before Christmas.
The Association of Chief Police Officers (ACPO) predicts that 28 days will not be enough time to question terror suspects in future as cases become more international, complex and technical.
In the summer when he had just taken over as Prime Minister and was considering his legislative programme, Gordon Brown probably banked on his popularity remaining in place long enough to steer the contentious issue through the Commons.
Haven't we been here before? This all sounds very familiar.
Indeed the government has been here before - and lost. In November 2005, Tony Blair suffered his first Commons defeat when he tried to get MPs to back extending the limit to 90 days.
The compromise reached was 28 days. In the process, ACPO was plunged in the centre of a political debate when it emerged senior officers had lobbied parliament for more powers.
Now ministers are hopeful they can raise the time limit up to a maximum of 56 days.
Is the government likely to get its way?
If enough assurances can be given to the civil liberties' lobby and sceptical back-benchers, then yes. It is likely that there would be far more judicial oversight if detention limits were extended. This may be enough to appease some critics.
Labour still has a healthy majority in the Commons and Mr Brown has not built up as much resentment from his own party yet as his predecessor.
However, the Conservatives and Liberal Democrats have vowed to fight the move, not least because neither police nor the government has yet come up with a firm example of when more than 28 days was needed. So far, most of the reasons cited have been hypothetical.
What is the alternative?
The government already has the power to detain suspects 30 days beyond the present 28 days if a state of emergency is declared across the UK. This provision exists in the Civil Contingencies Act.
Ministers argue that this could spread panic, however, and that an extension of the detention limit could be better.
What is the scale of the threat?
More than 1,000 people have been arrested for terrorism-related offences since the 9/11 attacks. At least 2,000 people in Britain have links to terrorists now, up by 25 per cent from a year ago, Jonathan Evans, the head of the UK's domestic spy agency MI5, said on the eve of the Queen's Speech.