Cavan and Jamie Hanna were, on paper, fairly unsuccessful property developers, living with their wives and children in townhouses in rural Kent.

Jamie's annual income was £15,000; Cavan told the Inland Revenue he earned a more handsome, but hardly dizzying, £50,000. What the brothers didn't declare was that between them they ran one of Britain's biggest drugs empires, earning up to £120m a year in a professional operation that has all the hallmarks of a storyline from the US crime series The Wire.

For 10 years, the pair used safe-houses across south-east England to store and then sell wholesale amounts of cocaine, ecstasy and amphetamines to street-level dealers. They wouldn't sell you less than £300,000 worth.

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They evaded capture for more than a decade, earning an estimated £1bn, by keeping their criminality close-knit. The brothers used an uncle to store cash for them, a cousin to courier money, and an old school friend to look after the safe-houses.

The pair, along with six others, were finally arrested last year and are set to receive lengthy prison sentences today.

Their undoing, as the druglords in The Wire know all too well, was covert surveillance photographs and hidden microphones, which recorded conversations between the gang in their cars.

The investigation began in 2007 when officers from the Serious Organised Crime Agency (Soca) identified the Hannas' cousin Stephen Mee, 51, as a possible drugs suspect.

But they stumbled on the Hannas by chance. Cavan Hanna, 39, who regularly met Mee, had a previous conviction for drink-driving so when he was spotted in a car, police decided to stop him and search the vehicle. Inside it, police found a booklet containing details of what they thought were drug deals but with this as their only evidence, Hanna was not arrested.

Instead they decided to widen their surveillance. In a taped conversation, Cavan Hanna told Stephen Mee that he was in the clear and so continued his drug dealing.

More surveillance showed that the gang's main safe-house was at a block of flats in Acton, west London. There, in October 2007, officers spotted Jon Bastable, 37, a childhood friend of the Hannas, handing a hold-all to Raj Koli, a man recruited to launder money for the gang. Koli, 33, who had seemingly ignored the preference of the Hannas to maintain a low profile and only use modest hire cars, was stopped by police in his black Ferrari. The hold-all contained £414,770, sealed in plastic wrapping, which contained the fingerprints of Cavan Hanna. Koli was arrested but, in order to allow the operation to continue, Bastable was not.

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Sensing the authorities were closing in on them, the Hannas, like their counterparts in The Wire, "changed it up". They moved their safe-house and ordered Bastable to stop working. Soca officers temporarily lost them.

They did not find them again until April 2008 when they identified a penthouse flat in Alderman House, Greenhithe, Kent. The flat was in the name of Darren Rankin, who had been using motorcycles to deliver and collect cash from the Hanna operation.

On 10 June last year, officers for the first time saw the Hannas together with Rankin, 38, and Bastable – and decided to arrest them. When officers from Soca kicked down the door of the safe-house, they found the Hanna brothers and Bastable "knee deep" in £1.1m worth of bank notes.

Two days later, the officers went to another of the Hannas' safe-houses in Waterstone Park, Kent. In the garage of the property was a man-hole cover. The officers lifted the cover to find two safes which had been placed there in concrete. Inside them, they found £2.7m in cash. The money was again sealed in plastic but, like the cash found in Koli's car, it contained the fingerprints of a Hanna. This time they were Jamie's. The raid at Alderman House uncovered bills for a warehouse in Abbey Wood, south-east London. So, on 13 June, officers raided this property. There they found 15,000 ecstasy tablets and 80kg of amphetamine. The drugs were worth £1m.

CCTV footage from the building introduced the Soca team to Martin Winter, 46, another member of the gang. He had been hired by Mee and was the only member of the group to actually touch the drugs. Officers said he was "sacrificial" in the Hannas' plans.Finally, officers visited the home of the Hannas' uncle, George Webb, 61, in Eltham, south-east London.

Jamie Hanna, 38, had been spotted visiting the house and when it was searched, £650,000 was found in the water tank in the loft. In total £5,131,211 was recovered, but officers say it is the tip of the iceberg.

A Soca officer, who cannot be named for operational security reasons, said: "We have never seen a Mr Big of this magnitude and the fact that they have pleaded guilty says a lot: organised criminals of this level do not plead guilty unless the evidence is very daunting."