Police and MI5 faced a major dilemma when they decided to arrest the fertiliser terror cell.

They knew they had to intervene in the plot to protect public safety but could not risk arresting such a large number of British Muslim men only to release them all later without charge.

The police had already faced fierce criticism from the Muslim community in the wake of 9/11 for making a series of arrests under anti-terrorism laws which failed to result in any charges.

This anger would have been dwarfed by the criticism that would have followed if police had swooped on the fertiliser cell before they had enough evidence to charge anyone involved.

To arrest such a large number of British Muslim men on suspicion of plotting to bomb their fellow citizens, only for them to be released without charge, could have irreparably damaged trust between the Islamic community and police.

It could also have seriously hampered their ability to conduct anti-terror operations in the future.

A delicate balance had to be struck. Police had to be sure of their case, but could not risk leaving it too late to intervene in the bomb plot.

It is a dilemma they have faced many times since.

One police source said: "We cannot afford to take the sort of risks with public safety that waiting until the point of attack entails - we have to look to intervene earlier.

"We saw intelligence suggesting there was a group of young British men whose ambition was to build and detonate bombs here, apparently with the intention of inflicting as many casualties as possible.

"Because of the scale and the type of the threat, we could not take risks with public safety so the real balance was between gathering evidence and maintaining the confidence of the community.

"Back in 2002/03 we were often subject to criticism for making arrests and then having to release people without charge, or not necessarily charges under the Terrorism Act.

"We had a group of young British men, apparently with a plan to kill their fellow citizens. It was absolutely critical that before we intervened we would be able to gather sufficient evidence that would be unequivocal in terms of demonstrating that these people were involved in terrorism.

"If we had had to release such a large group of British Muslim men...the criticism we would have been subjected to would have been such that it would have severely constrained our ability to run counter-terrorism operations in the future.

"If we had had to release all these people, there would have been a very critical eye cast upon the way the operation had been conducted."

Police could not be sure when the cell was planning to launch its attack.

By the early part of 2004, the plotters had paid around ten times as much as the fertiliser actually cost to store it and officers feared they could be about to remove it to manufacture a bomb.

A police source said: "We cannot be precise about the timings but we had intelligence that they were planning to remove the ammonium nitrate from storage.

"They had spent £1,000 on storing it by the time of the arrests."

Officers were also aware that the group's leader, Omar Khyam, was apparently making plans to leave the country before the attack was carried out.

This tactic had been a feature of previous al Qaida attacks and rang alarm bells with senior counter-terrorism officers on the case.

One source said: "There was intelligence that suggested Khyam was about to leave the country before an attack was actually launched.

"This was a pattern we had seen elsewhere. We were aware of how al Qaida returned to previous methodologies."

The final decision to move towards the arrest phase of the operation was made after March 19 2004, when surveillance officers heard some "worrying" snippets of conversation among the cell.

Detectives asked themselves: "Are we really in control of this terrorist plot or is there something we are not quite sure of?"

One police source recalled: "There was conversation about something that was 'ready to go'. We could not be clear what this was referring to.

"A decision was made to move toward arresting these people to stop any threat to public safety."