- 16 updates
ITV News understands the lorry carrying the 15 passengers belongs to a firm called Thermotraffic and that it stopped in Calais before reaching the UK.
ITV News producer Sandi Sidhu has tweeted:
Police have said the vehicle "originated in mainland Europe" and that it "has been seized as part of our investigation".
Police say they were alerted to the presence of 15 individuals after people reported hearing "noises coming from the back of the lorry".
They previously reported that there had been 20 people onboard.
A German national has been arrested on suspicion of facilitating the unlawful entry of people into the UK after a lorry containing 15 people from Eritrea and Kashmir was discovered at a service station in Ilminster, Avon and Somerset Police said.
A reporter for Chard and Ilminster News has tweeted this photo purporting to show the closed service station at Southfield roundabout.
A large heavy goods vehicle can be seen in the background surrounded by police and ambulances.
None of the people found travelling in a lorry in Somerset are expected to require hospital treatment, a spokesman for South Western Ambulance Service has said.
Paramedics were treating 15 individuals - 12 males and three males - at the scene after they showed signs of dehydration.
Two ambulances and two rapid response vehicles responded to a call at 3:13pm this afternoon.
Avon and Somerset Police say they have stopped a lorry containing 20 people suffering from possible dehydration.
The vehicle was stopped at a service station off the A303 at Southfields roundabout near Ilminster, Somerset at about 1pm.
"Their health and welfare is paramount as we try to piece together with the immigration authorities exactly what has happened," a police statement said.