Bus passengers were left coughing and gasping for breath after pepper spray was set off.

A woman was taken to hospital following the incident on Stagecoach's Oxford to Blackbird Leys service, as it drove along Cowley Road on April 30, at about 11.15pm.

The bus company said the incident was believed to have been a prank.

Officers arrested a 20-year-old man on suspicion of assault, but released him without charge.

Oxford University student Sinead Mandlik, 18, said people started coughing at the back of the bus, where she believed the spray had been set off, and within a couple of minutes coughing fits had spread throughout the vehicle.

The driver realised something was wrong and evacuated the passengers near to James Street, to change buses.

The driver of the next bus called police, because passengers were arguing with three Asian men they blamed for the spray.

Miss Mandlik, who studies at Keble College, said: "Some people were very unsettled because they didn't know what they had been breathing in."

David Whitley, Stagecoach marketing manager, said: "The consequences of releasing such a spray on a crowded bus could have been more serious, especially for people with breathing problems, such as asthma.

"We don't believe this to have been a malicious attack. We think it was a silly prank that went wrong."