A farm worker accused of killing a woman in a road crash may have been coming down from cocaine at the time, a court has heard.

Henry Bett is the son of farmer Stephen Bett, the former Tory chairman of Norfolk Police Authority, who now serves as Norfolk's police and crime commissioner.

The 26-year-old, of Hall Lane, Thornham, Norfolk, denies causing death by dangerous driving at Peterborough Crown Court sitting in Huntingdon.

Prosecutor Simon Wilshire told jurors that Bett had been speeding in his tractor when he collided head-on with a Fiat people carrier driven by Rebecca Brown, 43. Her car was crushed and she died at the scene, despite her son's attempt to save her.

Police found evidence Bett had previously taken cocaine and may have been suffering the after effects when the crash happened just after 3pm on December 4 2013.

Mr Wilshire added: 'We say he was driving dangerously. Blood samples taken four hours after the collision found the defendant had ingested cocaine some 12 to 24 hours before.

'There is nothing to say he had ingested cocaine in the hours immediately before the collision but the after effects of cocaine use can include a lack of concentration and a slowing of reactions which we say may have contributed to this collision.'

The tests found traces of the compound BZE in Bett's blood which indicate cocaine leaving the system.

There was evidence Bett failed to see the car, despite there being clear visibility on the West Acre Road near Mrs Brown's home village of Castle Acre.

The court heard he was returning from working on a local farm and was driving a green Fendt tractor in the middle of the road. Mrs Brown was taking her son, Thomas, to a hospital appointment.

The collision happened on a stretch of road where the 60mph national speed limit applies. Tractors cannot travel faster than 20mph in such areas and an inspection of the tractor found its speed limiter was set at closer to 40mph.

Bett told police that the limiter had been set correctly and must have been altered during the collision. Investigators found no evidence of this, Mr Wilshire said.

Mr Wilshire said: 'Mrs Brown took action to give the tractor space to pass safely, pulling onto the verge.

'The defendant appears not to have seen her car, even though there was 100 metres of visibility.'

The driver's side of the Fiat was completely crushed.

Afterwards Bett flagged down passing motorist, Anthony Adams, and appeared to be in a distressed state, jurors heard. Mr Adams then saw the Fiat in a hedgerow.

Bett told him: 'There was nothing I could do. I was as far over as I could be.

'She came round the corner, straight into me and took my front wheel off.'

He later told officers he was 'full on the brake' when the collision happened, the court heard.

However, Mr Wilshire said a reconstruction found the tractor could perform an emergency stop within seven metres whereas it in fact took Bett some 45 metres before coming to a halt.

Mrs Brown's son attempted to resuscitate his mother and several other motorists stopped to help. But nothing could be done to save her and she was pronounced dead at 3.37pm.

The trial is expected to last three weeks.