Officers are alarmed at how the gangs are moving into the illegal trade and exploiting the hopes of many refugees of finding a better life in the UK.
Some 20 gangs charge the migrants from pounds 250 to pounds 15,000. The more expensive "packages" include the services of a solicitor to assist in claiming benefits, "safe" houses, and work in restaurants, farms and backstreet "sweat shops".
The NCIS's recently formed Organised Immigration Crime Section has also detected a growing market in forged and counterfeit documents. An NCIS spokesman said: "There is a UK end of the operation. You tend to get packages - people will pay for transportation and advice on benefits and approaching the Home Office.
"There is a system of `safe' houses for them to stay until they are distributed to other parts of the country where they will be found work."
Details of the rackets given by NCIS officers to The Independent provide the most comprehensive picture yet of clandestine immigration. The NCIS estimates that between 2,000 and 4,000 people are smuggled into Britain every month. The top four gangs are bringing in up to 250 people each month, they say. Lorry drivers are paid about pounds 500 to smuggle in a load of 20 people, according to police sources.
Home Office figures show that 8,000 people were caught entering Britain illegally in the past year - less than a third of the lowest NCIS estimate. A further 7,000 were caught coming into Britain hidden in lorries.
The scale of organised smuggling has become so severe that MI5 and MI6 have been assisting authorities.
The three main nationalities of organised smugglers are the Turks, the Chinese and Asians from the sub-continent - mainly Indians, but some Pakistanis and Bangladeshis. The biggest single group being smuggled are Kosovo Albanians.