Only 340 of 1,471 people arrested between 2001 and 2008 were charged with terrorism-related offences, Home Office says.
Two-thirds of those people arrested for terrorism in Britain since 9/11 have later been released without charge, according to Home Office statistics published today .
The figures show that 1,471 people were arrested for terrorism between 11 September 2001 and 31 March last year, but only 340 were charged with terrorism-related offences. So far, the courts have convicted only 196 of those charged with such offences.
Home Office statisticians said the total of those arrested who had been charged or convicted was similar to that for other indictable criminal offences.
The statistics also show that as of March 2008 there were 125 people in prison in England and Wales for terrorist-related offences and a further 17 for domestic extremism or separatism, mainly for animal rights protests. Most of the terrorist prisoners – 75 – described themselves as British, with 111 declaring themselves to be Muslim.
Home Office ministers said the figures on terrorism arrests underlined the "considerable success" the police and security services had had in disrupting terrorist networks.
"Wherever possible, we seek to prosecute those involved with terrorism. Where we can't prosecute, we seek to deport. And where we can't deport, we seek to disrupt," said the Home Office minister Vernon Coaker.
But a detailed breakdown of the figures shows a more complex picture. The figure of 1,471 arrests is for the number of people arrested under "section 41 of the Terrorism Act 2000 and other legislation", without any further breakdown as to whether they were suspected of actively plotting explosions or engaged in fundraising through credit card fraud.
The figures do give some breakdown at the stage of charging. A total of 521 out of the 1,471 people arrested under terrorism powers were eventually charged with some offence, while in 131 other cases some alternative action was taken, such as deportation.
The official breakdown of the 521 people charged shows 222 were charged under specific terrorism legislation. A further 118 were charged for other "terrorism-related offences", such as attempted murder or conspiracy to cause explosions. The rest were charged with non-terrorist offences such as forgery, theft, misuse of drugs or other, unspecified criminal offences.
The Home Office said most of those arrested spend only a short time in custody, with nearly half held for less than one day in pre-charge detention and 66% for under two days. Only six people have been held for the maximum 28 days of pre-charge detention, and three of those were released without charge when the time limit was reached.
When convictions are considered, the success rate totals 7% for those found guilty under specific terrorist legislation, with 102 of the 222 charged convicted by the courts. But this conviction rate rises to nearly 14% when a further 94 convictions are included for terrorism-related offences such as conspiracy to cause explosions.
The Home Office added that the figures excluded a number of high-profile trials, such as the airline plot, where people were charged before 31 March 2008 but their *trials had not yet been completed.
Both the rate of charging and of convictions has remained broadly stable now for each of the seven years since 9/11 covered by the Home Office figures.