Police said in a statement that the incident at the Weeford toll plaza in the West Midlands, around 116 miles (187 kilometers) north of London, is ongoing. Police declined to give any more information, and the circumstances of the closure are unclear.
Megabus, the company that operates the bus, confirmed it was helping police with regard to "an allegation made against a passenger `' on one of its buses en route from Preston to London. It said the bus had 48 passengers booked for travel and was due to arrive at London's Victoria Station at 0955 GMT (5:55 a.m. EDT).
The BBC, citing unidentified sources, reported that the incident involved a passenger who poured a substance into a box, causing fumes.
A number of police and emergency vehicles were visible at the scene, where the bus was pulled over onto a shoulder.
Aerial footage of the scene showed what appeared to be passengers sitting on the ground in a marked-off square, spaced apart from each other.