A traffic police officer has been found guilty of careless driving after he crashed into another vehicle on his way to deal with a diesel spill.

PC Jacob Marshall was driving a police Volvo at more than 140mph just before he clipped the private car on the M9.

The accident happened when a Vauxhall Zafira pulled out to allow traffic joining the motorway from the M876 to come off the slip road.

Sheriff William Gallacher said Marshall drove "below the standard of a careful and considerate driver", adding he did not understand why the officer had not slowed down "even a bit" as he approached the busy junction.

He said: "Having seen the traffic coming over the bridge and the junction coming up, and the driver in the inside lane ahead of him, whey didn't he slow down even a bit?"

Marshall, 31, was originally charged with dangerous driving, but was convicted of the lesser offence after a two-day trial.

At the time of the offence on August 29, 2010, he was answering a call to a diesel spill on the M80.

Falkirk Sheriff Court was shown a which video showed him driving at speeds of up to 149mph on the motorway.

Marshall swerved on to the gravel trap as he tried to avoid the Zafira, but clipped the wing mirror at 121mph.

Off-duty taxi driver Fraser Lenny, 49, who was travelling with an 80-year-old passenger in the Zafira, said he saw no sign of the police car behind him until seconds before the collision.

He said: "I looked in my mirror before I pulled out, then looked again as I pulled out.

"I just saw blue flashing lights at the last minute, then a bang, and my mirror was off. I was shaking like a leaf."

The officer told the court he decided not to slow down because he didn't believe Mr Lenny was going to pull out into the outside lane.

He said: "There was no requirement for me to reduce my speed as I thought that the road was safe and clear. If I had any doubts at all about it I would have held back."

Superintendent Michael Cleary, 48, a specialist training in advance police driving, told the court that PC Marshall's driving was not in line with police training.

He said Marshall should have slowed down as he approached the slip road and ought to have noticed cars travelling over a bridge that were about to join the motorway.

He said: "I don't know what decisions the driver has made, but he has not been able to stop in the distance required, so his driving is contrary to the training."

Sheriff Gallacher deferred sentence for a fortnight to allow Marshall's defence to prepare its statement in mitigation.