Image caption Five of the men were arrested at Veolia's Chiltern Street depot in west London

Six men arrested by police investigating an alleged plot to attack the Pope have all been released without charge, Scotland Yard said.

The six - who work as street cleaners in Westminster - were arrested after they were allegedly overheard in the works canteen discussing an attack.

Police refused to confirm reports that the men were joking, saying they had a duty to investigate.

Policing for Pope Benedict's four-day UK visit is expected to top £1m.

The men are employed by Veolia Environment Services, a cleaning company contracted by Westminster Council.

At least five of the men are not British nationals. Most are thought to be Algerian.

The men, aged between 26 and 50, were arrested on suspicion of the commission, preparation or instigation of acts of terrorism on Friday, and released late on Saturday and early on Sunday.

Threat level

Armed officers arrested the first five men at the company's Chiltern Street depot in west London as they were preparing to go on shift. A sixth man was later taken into custody.

Police officers searched eight homes in north and east London and two business premises in central London, including a street cleaning depot.

Searches of the premises were completed and had not revealed any weapons or suspicious materials.

In the statement, the Metropolitan Police said policing arrangements for the papal visit were reviewed following the arrests, and that police were satisfied that the current policing plan for the Pope's visit remained appropriate.

"The itinerary has not changed. There is no change to the UK threat level," the statement added.

The current official threat level in the UK is "severe", which means that security chiefs believe a terror attack is "highly likely".