Thirty nine people have been found dead in a lorry.
The discovery was made in Essex, it is understood that the driver of the lorry has been arrested.
Officers were called by the ambulance service shortly before 01:40 BST after the discovery at Waterglade Industrial Park in Eastern Avenue, Grays.
The bodies are believed to have come from Bulgaria and entered the country at Holyhead on Saturday.
The lorry driver is understood to be a 25-year-old man from Northern Ireland, he has now been arrested on suspicion of murder.
Early indications are that 38 adults and one teenager have died, Essex Police said.
Ch Supt 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.”
The home secretary, Priti Patel, said: “I’m shocked and saddened by this utterly tragic incident in Grays. My heart goes out to all those affected.
“Essex police has arrested an individual and we must give them the space to conduct their investigations. Home Office immigration officials will be working closely alongside them to establish how this horrific event came to happen.”