Eighteen people who are believed to have entered the UK illegally were found by police stowed away in a lorry after a member of the public reported seeing suspicious activity in the vehicle.
A spokesman for Hertfordshire Police said the driver of the lorry - a 40-year-old man from Poland - was arrested on suspicion of assisting people entering the country unlawfully.
A member of the public called police shortly after 4pm on Saturday to report suspicious activity on a lorry on the M25.
That person followed the lorry as it made its way onto the M1 before police stopped it close to Junction 9 of the M1 in Flamstead, Hertfordshire.
The police spokesman said: "Eighteen people who are believed to have entered the UK illegally on the lorry were taken into custody for their safety."