Colombia is one of the world's largest producers of cocaine

Officers are also thought to have picked up 55,000 ecstasy tablets, 15kg of amphetamines and �7m of cars, jewellery, property and cash in a painstaking investigation.

They were attempting to smash a Colombian drugs and money cartel.

The seizures came after the National Crime Squad (NCS) swooped on addresses in north London and Essex during two raids in July and August.

The BBC's crime correspondent, Neil Bennett, says if the bonds are genuine the find would be by far "the biggest seizure of criminal assets in British legal history".

The bonds could be confiscated by the courts under the Proceeds of Crime Act, he said.

International arrests

News of the action only emerged on Thursday after the Colombian authorities, who worked with the British police in the operation, announced details of the raid.

BONDS An IOU issued to a lender by a borrower Bonds are sold with a promise that, at a fixed point in the future, the issuer will pay back the face value - in the meantime, the lender receives interest Bonds worth hundreds of billions of dollars are traded every day in the world's markets More on bonds

The vast find is three times more than the $2bn estimated worth of former Colombian drug baron Pablo Escobar as reported by Fortune Magazine in the late 1980s.

Nine British men charged with conspiracy to import cocaine and conspiracy to supply ecstasy appeared in court in the UK over the summer. One is understood to have a house in Colombia.

They are due to appear at Winchester Crown Court in November.

Four more people were also arrested in Britain, although they have been released on police bail.

And four further international arrests were made, two more in Colombia and two in Ecuador.

This is a very important victory, because it represents the most overwhelming blow against drug trafficking and money laundering Guillermo Anibal Ortega, Colombian investigator

A spokeswoman from the NCS said Southampton-based officers, assisted by Customs and Excise, stumbled across the US bonds during a drugs raid on a house in July.

She said: "We went in there looking for drugs, searching for drugs - anything other than that is a bit of a bonus."

A Home Office statement congratulating the NCS on the arrests also warned of the wider fight against drugs.

"Breaking down the supply chain is only half the battle," it said.

"We are cutting demand by targeting drug-users and getting them into treatment, spending more than half a billion pounds this year on treatment alone.

"By reducing demand, we reduce the impact of drugs on people's lives and cut crime."

Covert operation

News of the raids was apparently not released to help the Colombian authorities with their investigations.

A Colombian official said information provided to Britain by the country's Technical Investigation Department allowed the seizures.

Guillermo Anibal Ortega, director of the Colombia's attorney general's office investigative unit, added: "This is a very important victory, because it represents the most overwhelming blow against drug trafficking and money laundering."

The drugs were allegedly sent from Colombia to Europe via countries such as Ecuador and Mexico.