A Richmond farmer appeared before a public inquiry in Leeds last Friday in a test case which could have massive repercussions for the industry.

The inquiry centred on whether modified tractor units used by farmers on public roads for hauling were classed as agricultural or heavy goods vehicles.

Mr Ben Barker of Manor House, Whitwell, operates two adapted HGV units where the fifth wheel coupling has been removed and replaced by spraying equipment.

The units are registered as agricultural vehicles, meaning they pay a lower rate of road tax to HGVs, can run on red diesel and do not require either an HGV operator's licence or driving licence. But under the law, they must be mainly used off-road for farming purposes.

Mr Barker, who holds an HGV operator's licence, claimed the units were used around the farm, but accepted that they were occasionally used for hauling pig feed on round trips of up to 120 miles.

He was called before North-East area traffic commissioner Mr Mark Hinchliffe after the vehicles were involved in three separate road accidents last year.

In all three cases, the trailers they were pulling tipped over after the tow bar sheared.

The most recent accident occurred at the end of October outside Brompton on Swale, near Richmond.

Vehicle examiner Mr Karl Hunt said the trailer coupling had sheared and that there was extensive damage to a nearside brake drum where the wheel nuts had gradually come loose.

Mr Barker had sent another trailer out to pick up the load and Mr Hunt had found it to be unroadworthy as it had two bald tyres.

Mr Hunt suggested that the trailers were tipping over because the modifications to the tractor unit had altered how the trailers were attached, meaning the load was distributed unevenly.

The inquiry also heard that Cleveland police had written to Mr Barker in December, after one of his modified vehicles was stopped in a routine roadside check.

The inspector of the road policing unit wrote that the vehicle should be registered as an HGV and warned that if it was stopped again and found to be still registered as an agricultural vehicle, the force would prosecute.

Transport consultant Mr Paul Carless, representing Mr Barker, argued that the police, the DVLA and Customs and Excise were all aware that the vehicles were registered and taxed as agricultural units. He said his client had never been prosecuted for any motoring offences to suggest otherwise.

Traffic commissioner Mr Mark Hinchliffe said: "I cannot see why parliament would create a loophole to allow an adapted artic, capable of doing up to 60mph, to get out of the scheme when it is adapted so it is less stable than an articulated unit and can endanger the public. I think that is a preposterous approach."

Mr Hinchliffe will have to decide whether the units fall within the legal definition of an agricultural tractor or if they should be registered as HGVs.

If he finds they are not farming vehicles, the ruling will have huge implications for farmers across the country - and Mr Barker could face several criminal prosecutions.

The inquiry was adjourned until later this year.