Solicitors within the companies are suspected of providing a respectable image for worldwide crime syndicates to dispose of millions of pounds, mainly from drug deals but also from gun-running and contract killings.

The companies based in the City of London have become centres for the illicit gains of Colombian drug cartels and gangs in the US, Eastern Europe, Italy and Britain, the NCIS believes.

Simon Goddard: Money laundering is increasingly sophisticated

Some of the firms are thought to be well-known names, but officers have declined to name them.

Simon Goddard, a spokesman for the NCIS said that the use of legitimate companies by criminals was a sign of the times.

"Anywhere were organised crime has a presence there are lawyers who are feeding the money laundering," he said.

Mr Goddard stressed that the lawyers under investigation "know who their clients are and how their clients make their money, and they know it is not from a legitimate activity."

Under UK law, any suspicious money transactions have to be reported to the NCIS.

Meanwhile magistrates in central London remanded five men in custody for a week on charges relating to money laundering, Customs and Excise said on Monday.