The government said it would "disrupt" terrorists when it could not prosecute
Just 13% of people arrested under Britain's terrorism laws since the 9/11 attacks were convicted of terror offences, Home Office figures show.
Statistics reveal 56% of 1,471 terror suspects detained between September 2001 and April 2008 were not charged.
Of those arrested, 102 were convicted of terror offences and 94 were convicted on terror-related charges.
The Home Office insisted that the proportion of terror charges to arrests was similar to other offences.
'Real and serious'
Shami Chakrabarti, director of human rights group Liberty, said it was "worrying" that "the overwhelming majority" of those arrested were not guilty of any charge and half were not charged at all.
Massoud Shadjareh, chairman of the campaign group the Islamic Human Rights Foundation said it was an indication of "heavy-handed policing".
He also said the Muslim community had been alienated and criminalised in the name of security.
According to the figures, 521 of the terrorism arrests, or 35%, resulted in a charge, of which 340 were terror-related.
The number brought before the courts and convicted on terrorism-related charges, including conspiracy to murder and explosives offences, was 196.
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The figures compiled by the Office of the National Coordinator of Terrorist Investigations are the first of what will become quarterly bulletins published by the Home Office.
More than half of the people arrested since 9/11 on suspicion of terrorism were released without charge, which may fuel criticism that the police have used their powers indiscriminately. But the Home Office points out that over a third of those arrested have been charged with a crime, which is similar to the rate for all indictable offences.
The data demonstrates the difficulties facing prosecutors. However, the authorities can point to how some of what the public might view as the most dangerous terror suspects have been convicted under non-terrorism legislation, including conspiracy to murder or cause explosions.
From the total arrests, 56% were not charged and of the remainder other action was taken, including cases being dealt with on immigration matters.
Home Office minister Vernon Coaker said the 196 convictions highlighted the "real and serious threat from terrorism" faced by the UK.
"That is why the government is committed to investing in our counter-terrorist threat and wherever possible seeks to prosecute those involved with terrorism," he added.
"Where we can't prosecute, we seek to deport, and where we can't deport, we seek to disrupt."
Professor Paul Wiles, the Home Office chief statistician, said that the number of convictions was not a "complete record" as not all serious cases had yet been dealt with in the courts.
He said: "The conviction rate against arrests is quite likely under-recording what the eventual rate will be.
"People tend to think that when someone is arrested it is the same as being charged, but that is not the case."
At 31 March 2008, there were some 125 terrorist prisoners in England and Wales.
Of these, 62% were UK nationals and 91% classed themselves as Muslim.
According to the figures, there were 231 terrorism arrests in 2007/8 compared with a yearly average of 227 since 1 April 2002.
Of those arrested under section 41 of the Terrorism Act 2000, 46% were held in pre-charge detention for one day and 66% for less that two days.
The figures also show that six suspects were detained for a full 28 days since the maximum period of pre-charge detention was raised in July 2006. Of the six, three were released and three were charged.