Thirty-nine bodies were found in a lorry container in an industrial estate 32 kilometres east of London on Wednesday, prompting an outcry against people smuggling in the UK.
British police have launched a murder investigation after the discovery at the Waterglade Industrial Park in Grays, Essex, in the early hours.
Emergency services were called out but all 39 people, including a teenager, in the container were pronounced dead at the scene.
The driver of the lorry, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder. He has not been charged or identified.
Essex Police's deputy chief constable, Pippa Mills, described the incident as "an absolute tragedy and very sad day for Essex Police and the local community".
She said it could take a “lengthy process” to identify the victims and where they are from.
Essex police set up a casualty bureau with a hot line for people to call if they were concerned about relatives, to try to identify the victims.
Police initially said they believed the lorry had come from Bulgaria, entering the UK from Holyhead in Wales on Saturday. Holyhead is one of the main ports for ferries from Ireland.
But they later said that information was incorrect and the truck had travelled from Zeebrugge, Belgium on a ferry arriving at the port of Purfleet on the River Thames in eastern England.
It docked in the Thurrock area, near Grays, about 12.30am on Wednesday for 35 minutes before departing, Essex Police said.
CCTV footage published by UK media showed the lorry being driven towards the industrial estate where it was found at 1.10am. Police were called to the scene at 1.40am.
As bystanders gathered around the scene of the discovery, police announced that the lorry container with the bodies inside would be moved to a secure location in nearby Tillbury Docks.
Ms Mills said the move would allow the bodies to "be recovered while preserving the dignity of the victims".
UK Prime Minister Boris Johnson said in Parliament that he was appalled by the news and was being kept updated by the police.
"We know that this trade is going on," Mr Johnson said. "All such traders in human beings should be hunted down and brought to justice."
Police have not formally connected the incident with people smuggling.
The MP representing the constituency surrounding Grays described the news as "sickening" on Twitter.
"To put 39 people into a locked metal container shows a contempt for human life that is evil," Jackie Doyle-Price told Parliament.
"The best thing we can do in memory of those victims is to find the perpetrators and bring them to justice."
Opposition Labour leader Jeremy Corbyn said the deaths were an unbelievable tragedy that needed answers.
"Can we just think for a moment of what it must have been like for those 39 people, obviously in a desperate and dangerous situation, for their lives to end, suffocated to death in a container?" Mr Corbyn said.
Bulgarian authorities confirmed the lorry was registered in the country.
"The Scania truck was registered in Varna [on the east coast] under the name of a company owned by an Irish citizen," a spokesman for the Bulgarian Foreign Affairs Ministry said.
"Police said that it is highly unlikely they are Bulgarians."
Police said they believed the trailer of the truck had originated in Ireland.
The Irish Prime Minister, Leo Varadkar, told Parliament that an investigation would be undertaken if it was established that the lorry had passed through the country.
“I think everyone’s thoughts in this House this morning are with those who are dead, those who have passed on and their families,” Mr Varadkar said.
Zoe Smith, spokeswoman for the Medaille Trust, which provides refuge to victims of slavery, said it worked with clients who had been trafficked to the UK through different transport methods.
"We had one client from China who was smuggled into the UK aged eight, and all he can recall of his journey was being in 'a moving room' for two weeks," Ms Smith told The National.
“What that was is anybody’s guess. My guess is a shipping container but we don’t know for sure.”
Wednesday’s discovery is the biggest tragedy involving illegal immigrants in the UK for nearly 20 years.
In 2000, 60 Chinese immigrants were found in the back of a lorry in Dover on the English south coast.
Fifty-eight of them had suffocated to death, while two survived.
The lorry driver, a Dutch national, was jailed for his part in an organised people smuggling operation.
A tragedy of that scale was not seen again in Europe until 2015, when 71 people were found dead in an abandoned lorry on an Austrian motorway.
The bodies on board were men, women and children from Iraq, Syria and Afghanistan.
The vehicle was thought to have been part of a Bulgarian-Hungarian people smuggling operation.