David Anderson QC concluded that there was no evidence of a plot and that the police had used their powers under the Terrorism Act 2000 “lawfully and appropriately”.

Six street cleaners working in Westminster for Veolia Environmental Services were detained by police on suspicion of plotting to harm the Pope during his visit to Britain last September.

The men, all North African Muslims, were later released by police without charge amid. It was reported that a joke shared between the men in their canteen had been misinterpreted.

“There is no reason to believe, with the benefit of hindsight, that any of the arrested men was involved in a plot to kill the Pope, or indeed that any such plot existed,” said Mr Anderson.

However, he warned that there may be the temptation in future to use anti-terrorism powers where there is no reasonable justification to do so.

He said: “There will be future temptations to use the TA 2000 powers in relation to individuals as to whom the necessary reasonable suspicions do not exist, particularly in the context of international high-profile events such as the London Olympics.

“Constant vigilance is required to ensure that the legal boundaries of those powers are respected, as they were in this case.”

The review into the police’s handling of the case was launched by the Government’s terrorism watchdog last November to consider whether police had used their powers appropriately in arresting the men or whether they could have dealt with the threat in a different way.