Dawn raids have uncovered thousands of pounds believed to be linked to the farming of cannabis.

Police have arrested a 40-year-old man and a 27-year-old woman in Holloway and a 39-year-old man in Tottenham on suspicion of money laundering offences.

Detectives traced more than £3million which had been transferred to Vietnam over four years through multiple bank accounts set up by the suspects.

They found a further £30,000 after searching the north London premises.

The raids were carried out as part of a national police crackdown known as Operation Keymer, launched this week by the Association of Chief Police Officers.

Nineteen police forces across England and Wales are carrying out raids over the next two weeks on addresses where they believe cannabis is being grown.

Police officers uncovered two cannabis factories in Barking and Dagenham yesterday, seizing more than 300 plants.

Officers were called to a Dagenham house on Sunday night after reports of people trying to break in.

When they entered they found several rooms full of cannabis plants.

Detective Inspector Neil Hutchison said he suspected the intruders may have been trying to steal the plants.

He estimated there were about 100 of them - enough to produce about £15,000 of cannabis every three months.

Police say they are currently shutting about two farms a day in the capital.

Cannabis cultivation has become a highly lucrative activity, with gangs making up to £30,000 in three months from a farm of just 200 plants.

The proceeds are feeding other crimes and illegal immigrants are sometimes trafficked to the UK to grow the plants, police believe.

In January, the government decided to keep cannabis as a Class C drug.

But on Monday Home Office Minister Vernon Coaker said: "Those who use and sell cannabis will face tough penalties - up to 14 years for cultivation and dealing."

Cannabis farmers can expect to be arrested, their businesses closed down and their assets frozen, was the message from Acpo as it launched Operation Keymer.

Allan Gibson, ACPO lead on tackling cannabis cultivation and Commander at the Metropolitan police, said:"Cannabis cultivation is an increasing problem which must be nipped in the bud.

"Operation Keymer will see police closing cannabis factories across the country, arresting those concerned and using the Proceeds of Crimes Act to attack the criminal profits being made.

"Cannabis cultivation is seen by criminals as a low risk, high profit industry, but this operation will send out a clear message that cannabis production is a serious offence and that offenders will be brought to justice."

With farms springing up in quiet residential areas, police are urging the public to keep an eye out for them.

Signs of a possible factory include covered windows, gardening equipment left outside or a pungent smell coming from the building.

Earlier this year, police in London began using thermal imaging cameras to reveal the locations of secret cannabis factories.

The hand-held devices detect the extreme heat caused by the lighting systems used in the intensive rearing of marijuana plants.