A gang that became Britain’s biggest importers of high grade skunk cannabis made so much money that they left tens of thousands of pounds to rot in hidden safes.

The three “directors” of the organized gang made “grotesque profits” of up to £200 million, so much that they were unable to spend it fast enough.

In an undercover surveillance operation involving police in Britain, France, Holland and Switzerland the gang were seen driving to Europe with £500,000 stuffed into sports bags. In Geneva they used the cash to buy expensive designer watches.

Another £11 million was handed to a crooked employee at a currency exchange in East London and officers believe this was wired into accounts in Pakistan and Dubai.

When Calais customs officers intercepted members of the gang heading to Spain they found hundreds of thousands of pounds stuffed behind the dashboard of their car.

Senior members of the gang were also seen meeting private bankers in Geneva to discuss hiding their money. Much of it was sent abroad and high-priced villas were bought all over the world. The gang also spent hundreds of thousands of pounds buying land and funding a development of 12 luxury villas in Lanzarote.

Detectives say they now plan to seize the gang’s profit.

Details of the immense drug operation can be reported today after the last members of the gang were convicted at Southwark Crown Court.

Terrence Bowler, 40, headed up the operation along with Mark Kinnimont, 40, and Peter Moran, 37, who all pleaded guilty at an earlier hearing to conspiracy to import controlled drugs and conspiracy to launder the proceeds of crime.

To friends and family they gave the impression that they were wealthy businessmen, driving their children to well-known private schools in London from their expensive homes on leafy suburban streets.

But each morning, police watched as they used mirrors to check under their cars to see if trackers had been placed on them.

Detectives from the Metropolitan Police’s Specialist Intelligence Section first became aware of the gang in June 2006 and over the next two years noted that they imported £70 million of skunk — up to 250kg (551 lb) every week — enough for 225,000 street deals.

Officers, who arrested the gang in November 2008, believe that the “sophisticated operation” had been going for several years before they began targeting the gang, making them up to £200 million.

Officers involved in Operation Karnak discovered accounting books and lists of dealers with what they had bought and what they owed written on loose pieces of paper.

The drugs were imported from Holland under the guise of a flower business and would then be bought in bulk by dealers from around the country to resell.

Ten members of the gang, including the three bosses, admitted conspiracy charges while another two were found guilty on connected charges.

They will all be sentenced for importing nearly 19 tons of skunk cannabis at a later date. The previous largest amount ever dealt with in a British court was nine tons.

The three directors had never been in trouble with the police before but were described as being “at the top of the organized criminal network”.

Detective Chief Inspector Steve Wallace, head of Scotland Yard’s Special Intelligence Section, said the gang had “cornered the market” for skunk in England.

“So much cash was being generated through this venture that we found cash rotting away in safes hidden under the floor of lock-ups they were using to store the drugs.

“It was just going mouldy, they had so much that they did not know what to do with it. We took the ruined money to the Bank of England as it was no good to anybody.

“They were making grotesque profits on the back of the misery this drug causes. They were involved in a trade that allowed them to enjoy lavish lifestyles.

“The three main men are at the top of the criminal organized crime network and to all intents and purposes were legitimate businessmen, all with an air of respectability but we never saw them do a days work.”

Police continue to hunt one suspected gang member, Anthony Mills, of Beckenham, southeast London, who has disappeared, possibly abroad.