About 200 migrants from the Calais area are being smuggled into Britain in trucks each week, French officials and security sources said.
The estimate - equivalent to more than 10,000 illegal migrants arriving each year - represents a surge in the number of so-called "lorry drops", when migrants hiding in the back of goods vehicles jump out after safely reaching the UK.
Official figures show a continuing rise in the number of migrants found in the backs of trucks and cars in Britain, which more than doubled to 6400 last year, the chief inspector of borders and immigration said in a report last month. The new French estimate suggests that many more are coming through undetected.
Gangs of armed people-smugglers operating around Calais have started forcing trucks to stop before they reach the port so migrants can break in.
The motorway is regularly blocked with felled trees and debris by masked smugglers, often brandishing sticks and sometimes knives. French police responded to mounting political pressure by rushing 140 additional officers to the area at the weekend.
Security was tightened around the ferry port and the entrance of Eurotunnel last year after a spate of incidents that saw hundreds of people at a time attempt to enter both facilities. A few dozen migrants were estimated to succeed in making it to Britain each night at the peak of the incidents.
Efforts to enter the ferry and tunnel terminals tailed off with the introduction of new security measures, including a "moat" of flooded, low-lying land around the tunnel entrance. Consequently, migrants are now forced to travel further from Calais to stow away in lorries heading for the tunnel or cross-Channel ferries. "Each lorry is packed with dozens of migrants and many are stopped before they reach England," a regional security official said. "But we are certain that a few get through. The estimate is around 200 a week. We know others are crossing in small boats or vans."