Thirty-nine dead bodies have been found in a container in Thurrock in Essex, police have said.
Emergency services were called to Waterglade Industrial Park shortly before 1.40am this morning.
Early indications suggest 38 are adults and one is in their teens.
The lorry is from Bulgaria and entered the UK at Purfleet in Essex from Belgium, one of the main ports for ferries from Ireland.
A 25-year-old-man from Northern Ireland has been arrested on suspicion of murder.
Deputy Chief Inspector Pippa Mills told reporters today that identifying the victims remains the force's "number one priority".
She said police were in the early stages of the investigation and had not yet identified where the victims are from or their identities.
"This is an absolute tragedy and very sad day for Essex Police and local community," she added.
Chief Superintendent Andrew Mariner said: "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 enquiries continue."
Local employee Daniel Lake, who works nearby, told talkRADIO he used to see lorries carrying people when he worked on the docks.
He said: "Where I worked, it happened as well - it wasn’t a normal thing to happen but 39 in one trailer that’s odd, there’s a lot there. We used to see about three or four at most, 39 that’s a lot."
"The ones we saw were alive and they were uninjured as well – nothing like this," he added.
MP for Thurrock Jackie Doyle-Price addressed the issue in her constituency in today's Prime Minister's questions, calling for an international effort to "root these evil people out who are making and profiting from people’s hope and actually putting them into misery."
Home Secretary Priti Patel said it was a " truly shocking incident" and told the MPs the Home Office was "working collaboratively" with investigation teams including the National Crime Agency and Border Force.
She said the UK must "do the right thing as a country to ensure we uphold the right kind of values".
A cordon has been put in place and access to and from the industrial park remains closed.
The manager of a nearby cafe, Amy, told talkRADIO she arrived at work to find "wall to wall barriers" with three police cars at either end.
She said: "It’s horrible to think that’s literally just round the corner from where I am."
talkRADIO was at the scene
Essex Police has set up a casualty bureau for people to call if they are concerned about relatives.
It can be contacted on 0800 056 0944 for callers living in the UK, or 0207 158 0010 for callers dialling from outside the UK.
Anyone with information is asked to contact police on 101 or via the Essex Police website.