Passengers on a Megabus journey from London Victoria to Cardiff staged a revolt mid-journey when they spotted their driver falling asleep at the wheel.
Around 50 passengers refused to retake their seats when he stopped at a motorway service station.
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The Sun reports that they saw the bus driver's "head dropping" and the coach veered between lanes and almost hit a car.
Mum-of-two Jane Cohen, 46, told the Daily Mail: "Passengers saw the driver's head dropping as the coach swerved and vibrated across lanes.
"Women and children were crying in fear and trying to get the driver to stop. It was really scary and we thought it would crash."
She added: "The driver wanted to continue on up the M4 but we forced him to drive into the service station."
Police were called to Membury services and the driver passed a breath test.
A replacement driver continued the journey to Cardiff.
A spokesperson for Megabus told The Sun: "We understand concerns were primarily raised by one passenger. Police found the driver perfectly fit. CCTV and a monitoring system show they drove safely and without fault.