A gang produced "industrial quantities" of cannabis with a street value of more than £50 million a year after setting up a string of drugs farms.

The gang farmed drugs at locations in Kent and Norfolk between 2013 and 2015 and it is believed to be the biggest conspiracy to grow cannabis uncovered in the UK.

The Old Bailey heard, at the start of a four-day sentencing hearing, farms were discovered at industrial sites in Gravesend, Rochester, Sittingbourne, and North Walsham in Norfolk.

Each one was similar in set-up with two-storey wooden structures built inside industrial units and police found the equipment used was identical.

A total of 16,428 cannabis plants were seized from all the sites, which the prosecution said gave an estimated street value for a single year of between £25,418,160 to £51,852,840.

Dale Sullivan, prosecuting, said: "This case relates to a crime group who between 2013 and 2015 produced industrial amounts of cannabis at multiple locations around the South East and Norfolk.

"These cannabis factories when compromised by police contained thousands of cannabis plants and had the capability of producing an annual amount of cannabis that would have yielded a return measured in millions of pounds.

"This case captured defendants in all types of roles, those that are leading the enterprise along with those at the opposite end of the scale whose role of gardeners at various locations."

Referring to the value of the cannabis crop, he said: "On a single crop basis, the prosecution submits that the amount of cannabis produced at each of the seven production sites can properly be described as an operation capable of producing industrial quantities for commercial use."

The seizure from just one of the cannabis factories was also described as "the largest cannabis production seizure in the United Kingdom at that time".

The huge operation came to light after police uncovered one of the seven factories in Rochester, in June 2013, following reports of a burglary.

Officers entered and found the farm inside the industrial unit which had been leased by James Allen, since December 2012.

During their investigation police found Allen, along with other members of the group had rented three industrial units in the Canal Basin, in Gravesend, using false details.

Michael Edward Corcoran, his son Michael Jay Corcoran, Stuart Sollis and Stefan Henderson-Clarke, had all been connected with the site.

Officers were called to the area following reports of a disturbance in June 2017, and found Corcoran Snr in his Range Rover and a small cannabis plane on the floor nearby.

They then searched the site and four separate growing rooms with plants at different growing stages, and it was found cables ran to a second unit next door.

The next day officers found a larger cannabis factory in the other unit and then the following day the third drugs factory was found on the site.

Some of the defendants pleaded not guilty while others admitted their role in the conspiracy.

James Allen, 39, of Abbey Wood, London, pleaded guilty to five counts of conspiring to produce cannabis and three counts of abstracting electricity.

Stephen Macey, 25, of Paddock Wood, pleaded guilty to five counts of conspiring to produce cannabis and five counts of abstracting electricity.

Christopher Marsh, 30, of Margate, pleaded guilty to one count of conspiring to produce cannabis and one count of abstracting electricity.

Damon Mills, 18, of Margate, pleaded guilty to one count of conspiring to produce cannabis and one count of abstracting electricity.

The eight defendants were tried at the Old Bailey between September 5 and 28 November 2016.

Michael Edward Corcoran, 51, of Yalding, was found guilty of seven counts of conspiring to produce cannabis and five counts of abstracting electricity.

Michael Jay Corcoran, 28, of Gillingham, was found guilty of four counts of conspiring to produce cannabis and four counts of abstracting electricity.

Stefan-Alex Henderson Clarke, 27,of Walworth, south east London, was found guilty of four counts of conspiring to produce cannabis and four counts of abstracting electricity.

Stuart Sollis, 51, of Camberwell, south east London, was found guilty of five counts of conspiring to produce cannabis and four counts of abstracting electricity.

David Heath, 48, of Peckham, south east London, was found guilty of three counts of conspiring to produce cannabis and three counts of abstracting electricity.

Colin Walls, 52, of no fixed abode, pleaded guilty to one count of conspiring to produce cannabis and was found guilty of two further counts of conspiring to produce cannabis and three counts of abstracting electricity.

Terrance Murphy, 47, of Purfleet, Essex, was found guilty of one count of conspiring to produce cannabis and one count of abstracting electricity.

Stephen Creed, 45, of Great Yarmouth, Norfolk, was found guilty of one count of conspiring to produce cannabis and one count of abstracting electricity.

David Graham, 50, of Bromley, south east London, pleaded guilty to five counts of conspiring to produce cannabis and five counts of abstracting electricity just before the start of his trial at the Old Bailey.

During a second trial Alan Marquiss, 69, of Thamesmead, south east London, pleaded guilty to one count of conspiring to produce cannabis and one count of abstracting electricity.

Andrew Prowse, 32, of Camberwell, south east London, pleaded guilty during proceedings to three counts of conspiring to produce cannabis.

Fourteen of the gang are due to be sentenced later this week in a hearing which is expected to take four days.