A number of bureaux de change and currency transfer businesses are to be raided by police in the second week of July in a crackdown on suspected drugs money laundering. They say targeting the cash will reduce street violence linked to disputes between gangs by disrupting their activities.

Twelve businesses are being targeted in the capital on the first day of the operation, with inspectors visiting another 39 to check that they are complying with the law.

There are around 9,000 money exchanges in London, handling tens of billions of pounds each year between them – about a fifth of the UK market.

Detectives say the vast majority of these companies operate legitimately, but a “significant number” are involved in illegal activity.

The crackdown is a joint operation between the Metropolitan Police, HMRC and financial services watchdog the Financial Conduct Authority.

Det Ch Supt Mick Gallagher, who is co-ordinating the operation, said: “Money drives drug dealing, drives violence. The cash is the lifeblood of this. If you choke off the ability to trade effectively, then you disrupt the network.”

Following a similar operation in 2011, police described the use of legitimate bureaux de change by money launderers as a ‘weak point’.