Police found the bodies of 39 people inside a truck container believed to have come from Bulgaria at an industrial estate to the east of London on Wednesday.
The truck was thought to have entered Britain at Holyhead in Wales on Saturday and to have originally started its journey in Bulgaria. The driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder, police said.
"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."
Police made the grim discovery in the early hours of Wednesday at the Waterglade Industrial Park in Grays, about 20 miles from central London and close to docks on the River Thames. The 39 victims, 38 adults and one teenager, were all pronounced dead at the scene.
"Shocked & saddened by this utterly tragic incident in Grays," British Home Secretary (interior minister) Priti Patel said on Twitter.
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.