Essex Police were called by the ambulance service shortly before 1.40am today (23 Oct), after the discovery was made at Waterglade Industrial Park in Eastern Avenue, Grays.
Emergency services were called to the incident this morning, but sadly 39 people were pronounced dead at the scene. Early indications suggest 38 of the victims are adults and one was in their teens.
A 25 year old man from Northern Ireland has been arrested on suspicion of murder, police have confirmed.
In a statement, Chief Superintendent Andrew Mariner said, "This is a tragic incident where a large number of people have lost their lives. Our enquiries 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 this lorry is from Bulgaria and entered the country at Holyhead on Saturday 19 October and we are working closely with our partners to investigate."
A cordon has now been put in place and access to and from the Waterglade Industrial Park remains closed.
"We have arrested the lorry driver in connection with the incident who remains in custody as our enquiries continue," Mariner added.
"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."