The government faces a crucial test in parliament on Wednesday over its plans to extend the length of time that police can hold and question terrorism suspects before having to charge or release them.
Defeat in the vote would seriously undermine Prime Minister Gordon Brown, whose proposals have drawn widespread criticism from lawyers and human rights campaigners.
A poll conducted by YouGov for Wednesday's Daily Telegraph newspaper showed 69 percent public support for Brown's plan to extend the maximum detention period to 42 days "in exceptional circumstances". About a quarter (24 percent) oppose the plan.
Following are some of the issues surrounding the debate.
WHAT IS THE GOVERNMENT PROPOSING?
It wants to extend the maximum period of pre-charge detention, as it is known, from 28 to 42 days. However, it says the 42 days would only be used in rare cases when a "very grave and exceptional terrorist threat" exists. Any use of the new powers would have to be approved by parliament.
WHY DOES IT WANT TO INCREASE THE TIME LIMIT?
Al Qaeda-linked suicide bombers killed 52 people in London in 2005 and the authorities say they have foiled several other major plots. The official threat level remains "severe", defined to mean "an attack is highly likely". Investigations often require analysing huge amounts of seized computer material, which may be encrypted or in other languages, and suspects may have foreign links and use multiple false identities. All this, the government says, means a situation could arise where police have to release a dangerous terrorist suspect because they do not have sufficient time to collect the evidence needed to bring charges. It wants to act now before that happens.
WHAT ARE THE ARGUMENTS AGAINST?
Although police back the proposal, Britain's most senior prosecutor has said he has managed "quite comfortably" with the current 28-day limit. Civil rights groups say it violates basic freedoms to hold someone in prison for up to six weeks without charge when they may turn out to be innocent. They argue the bill is a draconian measure that will alienate people and may be exploited by Islamist militants who seek to radicalise youngsters against perceived injustice.
HOW DOES IT COMPARE WITH INTERNATIONAL PRACTICE?
Comparisons are difficult because of sharp differences between legal systems, including over the meaning of charging and pre-charge detention. In western Europe, police generally have several days to establish sufficient grounds for suspicion and hand the case to a judge or prosecutor for further investigation. But the suspect may remain in prison for months or even years while that investigation proceeds.
WHAT ALTERNATIVES ARE AVAILABLE?
Asked how the United States manages with a period of just two days' pre-charge custody, FBI chief Robert Mueller recently cited the use of phone-tap material as evidence in U.S. criminal trials. Britain is almost alone among Western countries in not allowing material from phone calls intercepted by the police or MI5 spy agency to be used in court. This means that while the authorities may have plenty of intelligence at the time of a terrorism arrest, they may have little or no admissible evidence so a prosecution case may have to be built from scratch.
Rights group Liberty has urged the government to remove the ban on intercept evidence in trials and to allow "post-charge questioning". This would mean that, even after charging people, police could re-interview them as further evidence emerges.
(Reporting by Mark Trevelyan and Kate Kelland; Editing by Matthew Jones)