Lorry drivers are being offered thousands of pounds to smuggle migrants into the UK via Kent, according to the National Crime Agency (NCA).
It says increasingly dangerous methods are being used by organised criminal gangs to conceal people on vehicles.
There is also evidence that a small minority of Eastern European lorry drivers are complicit with the gangs.
The Freight Transport Association (FTA) said gangs were "putting lives at risk" including those of lorry drivers.
Tom Dowdall, deputy director of the NCA, said the groups were becoming more sophisticated, with networks operating between France and Belgium and then into the UK.
"The principal way that people are being smuggled into the UK is in lorries across the English Channel.
"Because that's where the highest volume of traffic is, crime networks think they can simply lose themselves in high volume of movements," he said.
Mr Dowdall said lorry drivers could be offered anything from £2,000 upwards per migrant.
"In order for the organised crime groups to be successful they must have access to complicit lorry drivers to be able to move people across the Channel to the UK," he said.
"We've seen what we could describe as 'coffin concealments' - tiny areas within vehicles in which to hide and secrete people, and that includes women, children, as well as men."
In a statement, Natalie Chapman, head of South of England and urban policy for the FTA, said it was "hugely concerned" that criminal gangs were turning to more dangerous methods of smuggling people into the UK.