This was the amazing scene which confronted police inside a 1970s coach which had been converted into a drugs factory and parked up in an industrial estate in Dudley.

Officers found 600 plants spread across the rundown Bedford vehicle and three metal containers at the unit in Pedmore.

Specialist lighting and heating equipment had been installed to nurture the plants, which police say could have yielded 24kg of the drug, with a street value of £135,000.

Today, the men running the factory were beginning jail sentences of four years each.

An nPower worker stumbled across the factory at the Pedmore Road Industrial estate, Pedmore, following concerns that power was being diverted at a unit owned by Ian Walker, aged 44, and 39-year-old Paul Weston.

Wolverhampton Crown Court heard yesterday that the pair had leased the unit to another man for £20,000.

That man, who has never been traced by police, is said to have set up the factory, which was uncovered in April, 2008.

When Weston and Walker discovered the operation, he threatened them with a gun.

But after the third man vanished in November 2007, Walker and Weston were said to be ‘frequent visitors to the unit’.

Judge Martin Walsh said: “You were not simple gardeners, but you were the owners and managers of this operation – an operation which would have yielded substantial profits.

“After the third man had left, the two of you continued to operate the factory with a view to growing, collecting and selling the cannabis.

“Anybody involved in the large scale commercial sale or production of cannabis must expect significant sentences.”

Weston, of Lincoln Hill, Ironbridge, and Walker, formerly of Newquay Road, Park Hall, Walsall, but now of Worcestershire, pleaded guilty to production of cannabis.

Weston looked stunned and mouthed ‘Oh my god’ as the judge announced the sentence.

Lawyers for the men said they were not the “principal architects” of the factory, but had continued with an operation set up by the third man.

Netherton acting Sgt Bob Dalton, who investigated the case with Pc Dan Hall, said: “There was some suggestion that the electricity was being diverted and low and behold, there was a large cannabis factory there when we arrived.

“This has been a long and protracted investigation.”

Last month, West Midlands Police chief constable Chris Sims told the Express & Star of his concerns that cannabis factories were now being run by organised criminals.

The Mr Bigs of the industry pay footsoldiers, many of them Vietnamese inmigrants, to look after and nurture plants which can be sold on for huge profits.

All of the equipment necessary can be purchased online for a few hundred pounds.