Thirty-nine bodies were found on Wednesday near London in the container of a truck, which came from Bulgaria, UK police said.
A 25-year-old driver from Northern Ireland has been arrested on suspicion of the murder of the 39 people, they said.
The dead bodies were found in his truck in Essex, south-east England.
Essex Police said they were called by the local ambulance service shortly before 01:40 local time after the discovery at Waterglade Industrial Park in Eastern Avenue, Grays.
Early indications are that 38 adults and one teenager have died, Essex Police said.
The force said the truck came from Bulgaria and entered the country via Holyhead on Saturday.
Holyhead, located on the northwest tip of Wales, is one of the main ports for ferries from Ireland.
Chief Supt Andrew Mariner said officers were trying to identify victims.
He said: "This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened. We are in the process of identifying the victims, however, I anticipate that this could be a lengthy process."
A police cordon remains in place at the industrial estate where the discovery was made.
Meanwhile, Prime Minister Boris Johnson tweeted that he was "appalled by this tragic incident" and pledged to work closely with Essex Police to establish exactly what happened.
"I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened. My thoughts are with all those who lost their lives and their loved ones," he said.
Home Secretary Priti Patel said she was "shocked and saddened by this utterly tragic incident".
"Essex Police has arrested an individual and we must give them the space to conduct their investigations," the Indian-origin British minister said.
Thurrock MP Jackie Doyle-Price said it was "sickening news".
"People trafficking is a vile and dangerous business," she tweeted, adding that she hoped Essex Police could "bring these murderers to justice".
In 2014, as many as 34 Afghan Sikhs, suffering from severe dehydration, hypothermia and lack of air, were found alive inside a shipping container. One man was found dead, having passed away during the sea crossing from Belgium.
In June 2000, as many as 58 Chinese immigrants were reportedly found suffocated to death in the back of a lorry in Dover. There were two survivors.
A Dutch lorry driver was jailed the following year for manslaughter.