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.
News: We received several calls earlier this afternoon raising concerns about noises coming from the back of the lorry 1/3
Officers stopped the lorry at a service station on the A303 Southfields Roundabout at Ilminster at about 1pm 2/3
A total of 15 people were found on board including men, women and a 15 year-old boy. Full update to be issued on our website shortly 3/3
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.
The scene at Southfields Roundabout services. Police cordoned off a HGV. Appears to be around 20 people inside it. http://t.co/aUTBj5wVHG
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.
A mass ascent saw balloons take to the skies of Somerset as part of the Bristol International Balloon Fiesta.
Then and now pictures reveal the extent of damage floods had on the village of Muchelney in Somerset earlier this year.Read the full story ›
Prince Charles is set to return to the flood-hit village of Muchelney in Somerset.
The visit comes five months after the Prince of Wales saw the impact of flooding on agriculture and the livelihoods of people on the Somerset Levels.
The Prince used the emergency boat service - the only means of travelling in and out of Muchelney at the time - and a tractor to meet people in their own homes.
He also criticised efforts to help those affected by the floods, saying: "The tragedy is that nothing happened for so long."
A charity abseil down the side of a hospital has been postponed due to fears seagulls would dive-bomb fundraisers.
The 100-foot abseil, down Yeovil District Hospital, in Somerset, was due to take place tomorrow to raise funds for a new special care baby unit.
But organisers were forced to cancel the event when a seagulls' nest containing baby chicks was discovered yards from the launch site during final preparations.
A spokeswoman for the Flying Colours campaign, which aims to raise £500,000 for the unit, said the event would take place once the birds had moved on.
"The seagulls are fiercely protective of their offspring, and even during the inspection the inspectors were dive-bombed by the seagulls," she said.
Video footage shows emergency services at Wells Cathedral in Somerset after a woman fell 30ft inside the cathedral's bell tower.
The 60-year-old became trapped between two walls 150ft up before she was winched up to the roof by fire crews and airlifted to safety in a helicopter.