The number of migrants entering the UK in lorries is far greater than those crossing the Channel in dinghies, immigration officers have reported.
Lucy Moreton, general secretary of the Immigration Services Union, said preventing "lorry drops" was still the "day-to-day grind" for officers.
Six suspected migrants filmed climbing out the back of a lorry on the M25 were detained in Upminster on Friday.
A "major incident" was declared after 129 people entered by boat in December.
Home Secretary Sajid Javid vowed to re-deploy two Border Force vessels from the Mediterranean and the Royal Navy sent HMS Mersey to patrol the Channel.
Ms Moreton said: "The numbers coming over in small boats are only a small proportion compared to the number coming across in lorries.
"There is a lot of focus on small boats, but the day-to-day grind is still the lorries."
A BBC investigation has found that migrants trying to enter the UK by lorry are spreading their efforts from Calais to ports across Europe.
"We know there are large numbers massing in other ports," Ms Moreton said.
The number of so-called "lorry drops" - where migrants get out of an HGV after arriving in the UK - is not known.
More than 1,800 people were apprehended after making "clandestine entries" into the country through south coast ports in 2017/18, according to Home Office figures.
HGV driver Sam Hollamby filmed eight men leaving a lorry after it stopped near the Dartford Crossing at about 14:00 GMT on Friday.
"I saw a duvet being held out the side, I assume trying to catch the driver's attention," he said. "Then they started chucking glass bottles."
The Met Police detained six men at Corbets Tey Road, Upminster, at around 14:30 GMT. The men were transferred to Home Office immigration officers.