Thirty nine bodies have been found in a container in Thurrock in Essex, police said.
The bodies were found in a lorry shortly before 1.40am this morning.
Essex Police said it was called by the ambulance service shortly before 1.40am on Wednesday following the discovery of a lorry container with people inside at Waterglade Industrial Park, Eastern Avenue, Grays.
Some 39 people were pronounced dead at the scene. Early indications suggest 38 are adults and one aged in their teens, police said.
A 25-year-old-man from Northern Ireland has been arrested on suspicion of murder.
Chief Superintendent Andrew Mariner said: “This is a tragic incident where a large number of people have lost their lives. Our inquiries 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.
“We believe the lorry is from Bulgaria and entered the country at Holyhead on Saturday 19 October and we are working closely with our partners to investigate.
“We have arrested the lorry driver in connection with the incident who remains in police custody as our inquiries continue. I appreciate this cordon is going to disrupt the activity of local businesses in the area and we will work to ensure that disruption is kept as short as possible. We are working with Thurrock Council to mitigate against any impact our investigation scene will have locally.”
Paige Wade was driving past the industrial park at 4.15am on the way home from work.
She told the PA news agency: “I honestly didn’t see much, I had just finished work while driving up Motherwell Way and all you could see was blue flashing lights, as I was driving past there was police tape to cordon off the start of eastern avenue.
“I knew it was serious because of how many police cars and ambulances were there, but the police had parked their cars across the whole access of the road so you couldn’t see anything.
“There’s always lorries around there as they park up there for the night, I couldn’t say whether I did or didn’t see the actual lorry in question.”