So why did the government introduce a second act?

The Anti-Terrorism, Crime and Security Act (ATCSA) was passed as a response to the 11 September attacks.

Its most important section gives the home secretary the power to indefinitely detain without charge a foreign terrorist suspect if the individual cannot be deported for other legal reasons.

This was introduced because the government believed there were individuals in the UK who were a potential threat but it could not deport back to regimes known for human rights abuses.

How have these powers been used since 11 September?

As of October 2003 there had been 456 arrests since 11 September 2001 and five more under ATCSA.

Separate figures laid before Parliament show that as of summer 2003, 29 people had been charged with terrorist offences and 11 had gone to trial. Three people had been convicted of specific Terrorism Act offences.

Some of those arrested in counter-terrorism operations may have been convicted of other offences, such as fraud.

Under ATCSA, the home secretary authorised the detention of 15 foreign nationals. Two were released after voluntarily offering to leave the UK.

The remaining 13 have been pursuing appeals. These detentions remain the most controversial part of the counter-terrorism strategy.

How has anti-terrorism legislation changed over the years?

The now defunct Prevention of Terrorism Act was devised to deal with Northern Ireland. The original definition of terrorism was: "The use of violence for political ends [including] the use of violence for the purpose of putting the public, or any section of the public, in fear."

The new definition in the Terrorism Act 2000 is wider and covers behaviour here and abroad - even if the ends are not simply political and against the state.

Two examples cited when the bill was debated were militant animal rights activists and anti-abortionists in the USA.

So this is a more precise definition?

Ministers say terrorism comes in many forms but amounts to the same things - threatening or using violence to influence the public or lawful authorities.

But Liberty, the human rights law group, says anti-terrorism law now affects the right to protest.

This may sound academic, but the Metropolitan Police was accused of a "draconian" misuse of powers to stop and search against protesters outside an international arms fair in October 2003.

Under the legislation police officers can use the power "whether or not the constable has grounds for suspecting the presence of articles [which could be used in connection with terrorism]".

Some of the protesters searched have taken the case to the High Court.

So what are the checks and balances?

There are a number of checks built into the legislation - Section 44 searches must be authorised by a senior officer and approved by the home secretary.

Secondly, Lord Carlile QC conducts an annual review. In his latest report, he concluded the act was "working well given the difficulty of the issues with which it has to deal".

But in a separate report on the power to detain without trial, the Liberal Democrat peer said the men held should be kept in a "separate, secure environment with greater internal freedom of association and activity" because they had not been convicted of any crime.

A Privy Council panel is currently reviewing the provisions of ATCSA and is expected to publish its conclusions soon.

The crucial difference in checks and balances between these acts and the previous Prevention of Terrorism Act is that the old legislation has to go before MPs every year for approval. The new legislation will remain on the Statute Book unless Parliament chooses to repeal it.

How do our laws compare to those in the US and Europe?

The UK is the only European nation to have suspended article five of the European Convention on Human Rights which prevents arbitrary detention without trial.

Other countries have their own counter-terrorism laws and there has been a lot of effort at European Union level, largely led by the UK, to make sure the domestic laws all speak the same language.

In the USA, the Bush administration won near universal backing for the Patriot Act within weeks of September 11.

But the flagship anti-terrorist legislation has been so criticised since for curtailing civil liberties that US Attorney General John Ashcroft recently toured the nation to defend it.