Two adults and a child have been found in a trailer in Hampshire, which was being transported by truck from mainland Europe.
Police were called to water pump company Selwood in Chandler's Ford industrial estate on Wednesday afternoon after the driver raised the alarm.
Hampshire Constabulary said it was aware of the situation.
The three people are with immigration officials.
A statement from Selwood said: "A member of staff at the security gate alerted the police after the driver of a lorry which had stopped before entering the site raised concerns that there may be people in the vehicle.
"The lorry was due to make a delivery to Selwood.
"The matter was handled by the police and the lorry, which had been sent by a supplier from Germany, was taken on to the site at Chandlers Ford."