An oil leak into the engine is believed to be the cause of the dramatic bus fire in Marlborough High Street on Monday morning.

Stagecoach launched an investigation into the incident, which happened at 8.30am outside Lloyd’s Bank, and believe a hairline crack in a small part of the hydraulic oil pipe caused the leak.

The driver leapt into action when he was alerted to the engine fire by another motorist but was unable to put the blaze out using a fire extinguisher so rang the fire service.

There were no passengers on the bus at the time, although the driver was treated by paramedics for smoke inhalation.

Jayne Harper, who was walking through the High Street at the time, said: “It was like Armageddon, it’s not the kind of thing you expect to see before your morning coffee.”

People arriving at work were met with plumes of smoke as crews from Marlborough and Calne tackled the fire.

“It was a bit dramatic,” said Toby Hassell, store manager of Landmark.

“I was walking across the road and it was just a sea of fire engines, ambulances and police officers and I saw the diesel running across the road.

“I got here a little bit after the fire had been put out and we weren’t able to open the store initially so the fire service could do a dangerous gases check because of the smoke that had been emitted.”

Firefighters left the scene by 10.50am and the road was reopened by 11.45am.

A spokesman for Stagecoach said: “Safety is our priority and incidents such as this are very rare. Our vehicles are subject to a robust maintenance regime that is tougher than legal requirements and all buses receive the equivalent of an MoT every 21 days.

“We have continued to support the relevant agencies involved in responding to the incident. We will review the detail of the incident and take any further action required to promote safety.”

Diesel from the bus ran into the drains in the High Street which flow directly to the River Kennet.