LONDON, April 12 Now it turns out that the Pakistanis arrested on Wednesday on the suspicion of being terrorist plotters were actually identified as possible terrorists even before they left Pakistan.

The Independent on Sunday quoted security sources as saying that a number of those being held were identified as possible terrorist plotters by intelligence agencies before they left Pakistan, and were “allowed to run” to Britain through the student visa system, where they were tracked for several months.

A security source said the suspects had been watched since before January. One was allowed to enter Britain last week, even though there were irregularities in his paperwork. The man was told to return at a later date for an immigration appointment.However, since they were taken into custody in a rush there is a sense within the security services that now there may not be enough evidence amassed to build a case against them.

“And it may now take more than seven days for the investigators to reach their logical conclusions,” informed sources said.

Immigration Minister Phil Woolas last week denied that security checks for foreign students were inadequate, rejecting claims that the UK would not cooperate with Pakistani authorities over background checks on applicants for visas.

In fact, after the suspects had been identified in Pakistan, it would have been easier for them to be tracked through the student visa system, which would record their movements into Britain, rather than allow them to turn up in the country through other means and become lost in the system.

Raids were carried out in broad daylight on Wednesday afternoon after anti-terrorist chief Bob Quick allowed a secret document with details of the operation to be in full view of photographers as he left No 10, when he arrived for a briefing with Gordon Brown. He resigned from the force the next day.

Officials, scrambling to avoid a major row, immediately let it be known that the arrests had been planned for 2am Thursday morning, meaning the operation had been brought forward by a matter of a few hours. Yet security sources have told the newspaper that the Thursday raid was “only one option” and in fact the planned raid was possibly “several weeks” away.

Although Mr Quick resigned on Thursday morning as assistant commissioner, an intelligence source said he was already “out of his depth” on terrorism issues and it had been only a matter of time before he was replaced. Assistant Commissioner John Yates has taken over the role.

The current anti-terrorism mission, known as Operation Pathway, is being compared to Operation Crevice, which broke up a plot to blow up nightclubs with huge fertiliser bombs. In that case, intelligence officers waited until suspects obtained the fertiliser before carrying out raids. “It is probable that they (the suspected plotters in the latest investigation) were at the stage of only looking to source material,” one source said of the current operation.