Armed police have evacuated around 50 passengers from a coach on the M6 motorway near Lichfield, in the West Midlands. Reports suggest a man on the bus was seen pouring liquid into a smoking bag.
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Part of the M6 toll motorway near Birmingham is closed following a "police-led incident". Officers shut the road, near Birmingham, in both directions between junctions three and four, the Highways Agency said.
It is understood the incident involves a coach which pulled over on to the hard shoulder of the motorway of its own volition. The intercepted coach was a Megabus, which was heading south on the M6 toll road from Preston when it was stopped. Reports suggested a man was seen pouring liquid into a smoking bag on board the coach.
Armed police officers could be seen positioned next to the single-decker coach on the south-bound carriageway, as passengers were led off the vehicle one by one.
Eye-witness Nick Jones, who was stopped on the motorway for more than an hour and a half, told BBC News that police warned him to stay in his car, keep his windows closed and not to use air conditioning.
"I was beginning to feel a little uneasy," he said. "I was beginning to look around for an escape route."
A Megabus spokeswoman said: "We are assisting police with their inquiries into an allegation made against a passenger who was travelling on board one of our services."
A number of emergency service vehicles were parked at the toll plaza, about 200m from the coach. Seven police vans arrived at the scene shortly before 11am.