The covert system, fitted to criminal ringleader Robert Henson's car, enabled officers to record hours of discussions between drug dealers involved in the cannabis supply chain.

And despite Henson's best efforts to locate and destroy the device, it could not be found using conventional radio frequency detectors, Newcastle Crown Court was told.

Henson's "business" discussions in importing large quantities of cannabis from continental Europe and distributing it across Tyneside were recorded by investigating officers.

And when confronted with the covert material Henson, 59, of Shincliffe, County Durham, pleaded guilty to a charge of conspiracy to supply the Class C drug when his trial was due to start at Newcastle Crown Court.

The court was told police used the bugging device for a month in 2007 to gather evidence against Henson and his gang.

Tim Gittins, prosecuting, said on several occasions Henson discussed his fear that he was being bugged as he spoke to associates, but the detectors he bought could not find the device.

When police arrested Henson and his gang on September 10 they found more than 360 kilos of cannabis resin in cars, vans and lock-ups in Killingworth and Newcastle, as well as more than £20,000 in cash.

The cannabis resin, imported from Henson's Spanish contact known as Billy, could fetch a street value of tens of thousands of pounds, Mr Gittins told the court.

"It was clear that Henson was at the centre of a business in the north east, distributing large, whole-sale amounts of cannabis resin that had been delivered directly from the continent," said Mr Gittins.

Henson's right-hand man Peter Thirlaway, 38, of Dowling Avenue, Whitley Bay, along with associates Paul Thompson, 48, of Telford Close, Backworth, and John Rae, 39, of Killingworth Lane, Backworth, admitted the same charge.

Henson's daughter Amanda Elliott, 34, of Shincliffe, County Durham, pleaded guilty to a charge of concealing criminal property.

They will be sentenced at a later date.

Alleged co-conspirators Frederick Foreman, 55, of Monks Park Way, Longbenton, North Tyneside; Ian Dixon, 31, of The Briery, Newcastle; and Kevin Curry, 47, of West Road, Newcastle, deny a similar charge of conspiracy to supply cannabis and their trial continues at Newcastle Crown Court.