Police face criticism over operation but deny that bringing arrests forward disrupted investigation.
The government today faced a barrage of criticism after police
released without charge the remaining 11 suspects arrested a fortnight ago in the north-west of England over an alleged terror plot.
The last two men to be released joined nine others given their freedom last night and one freed on 11 April.
Opposition parties, human rights lawyers and Muslim groups accused the government of mistreating the suspects and botching the anti-terror operation.
The shadow security minister Baroness Neville-Jones said: "It is very worrying that, following an investigation based on strong intelligence into what the prime minister described as a serious terrorist plot, the police have not been able to present sufficient evidence to the Crown Prosecution Service on which it could lay charges against any of the 12 arrested."
Shami Chakrabarti, director of human rights group Liberty, said: "In the vital task of policing open societies, it is inevitable that you arrest more people than you charge and that sometimes suspicion will never be converted into evidence. But national security deportation is an extremely shadowy process and we need assurances from ministers that these powers will only be used for public safety and not for political signalling."
Gordon Brown had claimed the operation uncovered a "very big plot" against the UK.
In a statement, Greater Manchester police said: "The 11 men were questioned and the evidence gathered presented to the CPS who advised there was insufficient evidence gathered within the permitted timescales which would have allowed a warrant of further detention to be gathered or charges to be pursued.
"It is not possible to bail people under terrorism legislation so the men were released.
"Public safety is always the police's top priority and all information is fully considered and acted upon appropriately to minimise risk to the public."
In a press conference on the steps of the police headquarters, chief constable Peter Fahy said: "These people are innocent and they walk away … there are constant threats to this country but we totally respect the situation, we respect that they are innocent until proved guilty."
Fahy denied that there had been a dispute with the security services or that bringing the arrests forward by up to 12 hours had disrupted the investigation. He criticised speculation by outsiders, including retired officers, and said: "I have not conducted any speculation. I do not feel embarrassed or humiliated by what we have done because we have carried out our duty. I don't think a mistake has been made at all."
The BBC reported that security services continued to maintain that a terrorist plot had been disrupted by the operation.
Nine of the men are due to be deported after being handed over to the UK Border Agency but it was not immediately clear what would happen to the last two men. One of the 11 is understood to be a British national. The releases came after investigators spent 13 days searching for evidence following the arrests from a number of addresses in Greater Manchester, Liverpool and Lancashire under the Terrorism Act.
The police operation was condemned today by a spokesman for the Muslim Council of Britain. Inayat Bunglawala told Radio 4's Today programme: "When these arrests took place in very dramatic circumstances with students being pulled from universities and thrown to the floor, we were told by the prime minister, no less, that this was part of a very big terrorist plot. Clearly there just has not been the evidence produced to substantiate such a plot."
The raids led to the resignation of the country's leading counterterrorism officer, Bob Quick
, after he inadvertently allowed details of the operation to be photographed. Before the men had been interviewed the prime minister spoke of how the police had foiled a "very big plot", but as early as last Monday it emerged the government had spoken to Pakistani officials seeking reassurances that if the men were deported they would not be tortured.
The Guardian understands the decision to arrest the suspects on 8 April came after a three-way row between MI5, senior officers in the Metropolitan police and the Greater Manchester police. MI5 was strongly of the opinion that the arrests should wait while more intelligence was gathered. But in an example of the tensions between Whitehall counterterrorism officials and their counterparts in the police, the decision was made to take "executive action" even though the intelligence suggested there was little evidence to charge the suspects.
It is understood anti-terrorist officers in the Met disagreed with their counterparts in Greater Manchester that the arrests should be made. But the concern that there was a threat to the public led to the decision being made to move in.
Investigators had hoped to find something at the suspects' homes. But after initially hunting for, and failing to find, bomb-making equipment, they turned to the computers hoping that some evidence of a plot would turn up. They found nothing substantial.
Despite media reports and the plot being talked up by Brown, there was never any evidence that the suspects had identified targets for an attack.
The arrests came several hours earlier than the police had planned after Quick accidentally allowed a top secret briefing document on the raids, Operation Pathway, to be caught on camera by a photographer outside Downing Street, when he went to brief ministers on the action. The error led to his resignation after politicians condemned the security breach.
Officers from the north-west counterterrorism unit arrested 12 men under the Terrorism Act following the raids on 8 April. Of the 12 men initially arrested, 11 were Pakistani nationals, 10 held student visas and one was British.
A 12th suspect, an 18-year-old, was released without charge and handed to the Border Agency for deportation on 11 April.
The arrests led to claims that the student visa system contained loopholes which allowed abuse by people attempting to enter the country for illegal activities.
The government has admitted that the system is flawed and, two weeks ago, introduced tougher measures designed to root out false applications. At the time of the arrests counterterrorism sources expressed the fear that al-Qaida was using Pakistani students not known to the security services.
Sixty-eight people are currently on trial, or awaiting trial for alleged terrorist offences.
According to the Home Office, from 11 September 2001 to 31 March 2007, 1,228 terror-related arrests were made, excluding Northern Ireland.
Since January 2007, 92 people have been convicted in significant terrorist cases with 47 people pleading guilty.