By Dominic Casciani
BBC News home affairs reporter
The government is unveiling a major new package of proposed counter-terrorism laws. What are ministers putting forward and why are they doing it?
There have been four Acts dealing with terrorism since 2000
What are the government's latest proposals?
The most important proposal in the government's Counter-Terrorism Bill is to extend the time a suspect can be held without charge from 28 to 42 says.
Under emergency powers - normally only used in wartime - the government can extend detention for an extra 30 days.
But ministers want to be able to provide investigators with up to six weeks of detention without declaring a state of emergency.
Home Secretary Jacqui Smith is pledging safeguards including what she says is a degree of parliamentary oversight.
How will this power work in practice?
A chief constable and the Director of Public Prosecutions would go to the Home Secretary to request an extension up to 42 days.
Parliament would be notified within two days - but would not be able to block the use of the power at that stage.
What have ministers said on the issue previously?
In July 2007, Gordon Brown made his first detailed statement on anti-terrorism laws since becoming prime minister. He put forward four proposals on the detention of suspects, including the use of emergency powers.
The government suggested its preferred option would be to extend the current time limit by up to 28 days. This would have brought it to 56 days, although that figure was not mentioned by the prime minister in his speech.
That figure has now been ruled out and the Home Secretary is lobbying sceptical MPs one by one to support 42 days.
What do critics say about the plan?
The Conservatives and the Liberal Democrats are warning they will vote against the measure. When former Prime Minister Tony Blair tried to introduce 90 days detention, 49 Labour MPs rebelled, enough to defeat the government.
The Director of Public Prosecutions Sir Ken MacDonald has told MPs he has not asked for the increase to 42 days and is "satisfied with the position as it stands at the moment."
Former Attorney General Lord Goldsmith has expressed doubts.
Police chiefs concede they have not had a case where they have need to hold someone beyond 28 days, but have been forced to let them go.
What is the case for going beyond 28 days?
The government says the growing complexity and international nature of terror plots means police will need in exceptional circumstances more time to question suspects before charging them.
One example given by senior investigators relates to an alleged major terrorism plot yet to come to trial. In that case, say officers, they applied a special chemical treatment to a garage under investigation. The process took two weeks to reveal fingerprints.
TERROR DETENTION LIMITS
France: Up to 72 hours without seeing a lawyer and four years in pre-trial detention
Germany: Must be seen by a judge within 48 hours but can be held without trial during investigation
Greece: Up to 12 months - 18 months in extraordinary cases
Italy: Up to 24 hours without seeing a lawyer
Norway: Up to 48 hours - a judge can increase this period
Spain: Up to 72 hours without a lawyer - can be increased to a maximum of 13 days
USA: The attorney general can detain foreign suspects but must start deportation proceedings within seven days. Suspects can be held for periods of six months
These prints led to a tiny computer pen drive hidden in the rafters which detectives say is critical evidence.
How many people have been held to the current legal limit?
So far six people have been held close to the 27/28 day limit. Five of these people were arrested on 9 and 10 August 2006 in connection with an alleged plot to target airliners, a case expected to come before the courts later this year.
Two of the five were charged and three released before the deadline. The sixth case of 28 days detention involved Habib Ahmed, arrested and charged in Manchester in connection with unrelated allegations.
What other proposals are included in this bill?
Ministers want to allow the police to question a suspect after they have been charged.
Ministers want to make terrorism an "aggravating factor" in sentencing for people not convicted under counter-terrorism powers but clearly linked to an extremist cause. This would work in the same way as cases which lead to a criminal getting a stiffer sentence for an assault that is racially-motivated.
One key issue yet to be resolved is whether or not phone tap intelligence gathered by the security services will be allowed into criminal trials. A special Privy Council committee is looking at this issue.
Other measures include a proposed register of convicted terrorists, similar to that for sex offenders and powers to seize passports to prevent someone travelling abroad for extremist aims.
What anti-terrorism legislation currently exists?
There have been four Acts dealing specifically with terrorism since 2000. The Terrorism Act 2000 widened the definition of terrorism and created crimes of inciting or providing training for terrorism.
The Act allowed police to hold someone for up to a week with court approval. The legislation also banned organisations such as al-Qaeda.
What happened next?
Further legislation followed the al-Qaeda attacks on the US and it included powers on terrorist fund-raising and how police investigate terrorism.
Most controversially, it gave ministers the power to detain without trial foreign nationals who were suspected of terrorist links where there was insufficient evidence to prosecute.
That power was later dropped after the Law Lords ruled the detentions were discriminatory and not proportionate to the threat faced by the UK.
So where do control orders fit?
Control orders were the government's response to that defeat. These orders restrict a suspect's movement and behaviour and can be similar to house arrest.
Critics say suspects cannot mount a meaningful defence as they never know the full case against them.
What about the fourth act?
The Terrorism Act 2006 came in after the 7 July London suicide bombings. It was the source for the Commons row over 90 days detention, with MPs finally settling on 28 days. It also introduced offences including preparing a terrorist act and glorification of terrorism.