Just one person in eight arrested under anti-terror laws is eventually found guilty and the proportion was even lower last year, new research has disclosed.
The low conviction rates prompted calls last night from lawyers and civil liberties groups for the "draconian" and "overbroad" legislation to be overhauled. The coalition Government has promised to review its anti-terror measures, including the controversial power to hold suspects for up to 28 days before they are charged.
In the 10 years since the Terrorism Act came into force, 22 per cent of suspects arrested under its provisions were charged, and less than 13 per cent were subsequently convicted. The ratio fell further last year, according to the statistics compiled by the legal information service Sweet & Maxwell. In 2009 the number charged was 11 per cent and the conviction rate was down to less than 4 per cent.
Sweet & Maxwell said there have been 1,817 arrests under the Terrorism Act which have led to 402 charges – a rate of 22 per cent that compares with 31 per cent for all indictable offences among adults.
A total of 235 were found guilty of terrorism-related offences, a conviction rate of 12.9 per cent. There were 207 arrests in 2009, which led to 23 charges and eight convictions, although the latter rate could eventually increase because prosecutions are yet to come to court.
Stephen Grosz, head of public law and human rights at the law firm Bindmans LLP, said: "The concern has always been that most individuals arrested under the draconian powers of the Terrorism Act 2000 could have been dealt with just as effectively under other areas of criminal law.
"These statistics do suggest that the police may have been far quicker to make use of powers of arrest under the Act than was necessary."
But Lord Carlile of Berriew, the independent reviewer of terrorism legislation, said a relatively high proportion of suspects taken to court were convicted. He said: "It is inevitable that the comparison between arrest and conviction is rather different. In terrorism cases the police almost always arrest early because they cannot run the risk of a terrorist act being carried out."
A Home Office spokeswoman said: "There is a widespread concern that counter-terrorism laws have gone too far, that they are being misused and that they are eroding important civil liberties.
"We want laws that are effective but which are also proportionate, focused and transparent – that strike the right balance between security and civil liberties."