British police found the bodies of 39 people inside a truck container, believed to have come from Bulgaria, at an industrial estate in Essex.
The discovery was made on Wednesday after emergency services were alerted to people in the container, on an industrial site about 20 miles from central London.
The trailer arrived in Purfleet on the River Thames from Zeebrugge, Belgium, about 00:30 BST.
Ambulance staff discovered the bodies in the unit just over an hour later at Waterglade Industrial Park in Grays.
Police said the driver of the truck, a 25-year-old man from Northern Ireland, had been arrested on suspicion of murder.
The victims, 38 adults and one teenager, were all pronounced dead at the scene after they were found at the Waterglade Industrial Park, not far from docks on the River Thames.
Police said the tractor unit (the front part of the lorry) came from Northern Ireland and picked up the trailer from Purfleet.
The tractor and trailer then left the port shortly after 01:05 and officers were called around 30 minutes later when ambulance staff made the grim discovery.
A spokesman for the Bulgarian foreign affairs ministry said the truck was registered in the country under the name of a company owned by an Irish citizen. He said it was "highly unlikely" the deceased were Bulgarians.
Prime Minister Boris Johnson said he was appalled. "I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened," Mr Johnson said on Twitter. "My thoughts are with all those who lost their lives & their loved ones."
Police officers in forensic suits were on Wednesday inspecting a large white container on a red truck next to warehouses at the site. Police had sealed off the surrounding area of the industrial estate with large green barriers as they carried out their investigation.
"This is a tragic incident where a large number of people have lost their lives. We are in the process of identifying the victims, however I anticipate that this could be a lengthy process," said Chief Superintendent Andrew Mariner, from Essex Police.
"We have arrested the lorry driver in connection with the incident who remains in police custody as our enquiries continue."
Nearby businesses said they had been unable to gain access to their units on the site due to the large police cordon.
"The police came in the night - they have closed the whole area," said a worker at a nearby cafe, who declined to give his name.
In Britain's biggest illegal immigrant tragedy in 2000, British customs officials found the bodies of 58 Chinese people crammed into a tomato truck at the southern port of Dover.