Driver of the lorry arrested as authorities report victims include 38 adults and one teenager.
Police in south-eastern England say emergency services have recovered 39 bodies from a container truck believed to have been driven from Bulgaria.
Chief Superintendent Andrew Mariner said on Wednesday that authorities were in the process of trying to identify the deceased; 38 adults and one teenager.
Police were alerted by the ambulance service shortly before 1:40am local time (00:40GMT) following the discovery of a truck container with people inside at the Waterglade Industrial Park.
Mariner said the truck entered the country on October 19 and that police were “working closely with our partners to investigate”.
The 25-year-old driver, from Northern Ireland, was arrested on suspicion of murder, the police said.
UK Prime Minister Boris Johnson said he was “appalled by this tragic incident”.
“I am receiving regular updates from the Home Office and will work closely with Essex Police as we establish exactly what has happened,” he said. “My thoughts are with all those who lost their lives and their loved ones.”
Al Jazeera’s Paul Brennan, reporting from London, said the lorry entered the UK mainland at the port of Holyhead in North Wales, which is “the transit point for freights and lorries coming in from Northern Ireland”.
“A murder enquiry has begun,” he added, “but everybody’s thoughts will be with the bereaved relatives of the deceased.”
Essex Police said the people were pronounced dead at the scene in Grays, east of London. “This is a tragic incident where a large number of people have lost their lives,” the service said in a statement.
A cordon has been put in place and access to and from the Waterglade Industrial Park remains closed.
“The cordon around the scene is expected to be in place for quite some time,” Brennan said.
“The police are saying it will take a lengthy time to identify the victims,” he added.
SOURCE: NEWS AGENCIES