A murder investigation has been launched after 39 bodies were found in a lorry container on an industrial estate in Essex.
The victims, which include one teenager, were found at the Watergate Industrial Park in Thurrock, Essex Police said.
The force said they believe the lorry is from Bulgaria and entered the country at Holyhead, north Wales.
A 25-year-old man from Northern Ireland has been arrested on suspicion of murder.
“This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened,” Chief Superintendent Andrew Mariner said.
Thirty-nine bodies have been found in a lorry container on an industrial estate in Essex, police have said.
39 bodies found in a lorry container in Essex
Vehicle ‘from Bulgaria and entered the country at Holyhead on Saturday’, police sayThe victims were found at the Waterglade Industrial Park in Thurrock, Essex Police said.
Early indications suggest there are 38 adults and one teenager.
A 25-year-old man from Northern Ireland has been arrested on suspicion of murder.
The lorry is from Bulgaria and entered the country at Holyhead, North Wales, one of the main port for ferries from Ireland.
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 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.
"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."
Boris Johnson, the prime minister, tweeted: "I’m appalled by this tragic incident in Essex.
"I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened.
"My thoughts are with all those who lost their lives and their loved ones."
A witness who was driving past the industrial park at 4.15am on the way home from work said "all you could see was blue flashing lights".
Paige Wade said: "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."
Priti Patel, the home secretary, tweeted: "Shocked and saddened by this utterly tragic incident in Grays.
"Essex Police has arrested an individual and we must give them the space to conduct their investigations."
A cordon has been put in place and access to and from the Waterglade Industrial Park remains closed.
Thurrock MP Jackie Doyle-Price tweeted: "Sickening news of 39 people found dead in a container in Grays.
"People trafficking is a vile and dangerous business. This is a big investigation for Essex Police.
"Lets hope they bring these murderers to justice."
Footage from the scene shows a police cordon and tent set up near the lorry:
In a written statement, Priti Patel, the home secretary, 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."
DUP leader Arlene Foster tweeted: "Horrifying news coming from Essex this morning.
"Thinking of those who have impacted and their families during such a distressing time."
Diane Abbott, the shadow home secretary, tweeted: "Saddened to hear of the appalling incident involving the deaths of 39 people in the back of lorry in Essex.
"My thoughts are with the family and friends of those who have died. The Essex Police are thoroughly investigating this situation."
Bulgaria's foreign ministry said it could not confirm at this stage whether the truck had started its journey from the Balkan country.
"We are still checking the information, published in the British media and we're contacting the authorities," foreign ministry spokeswoman Tsvetana Krasteva said.
Essex Police have said they have not identified where the victims are from or their identities.
Deputy Chief Constable Pippa Mills said the discovery of the 39 bodies was an "absolute tragedy" and appealed to anyone with any information to contact police.
Deputy Chief Constable Mills added: "This is an absolute tragedy and very sad day for Essex police and the community.
"We will continue to work alongside many other partner agencies to find out what led to these deaths.
"I would like to appeal to anyone who has any information to contact my officers at the major investigations team."
Here is footage of the press conference at Grays police station, held by Deputy Chief Constable Pippa Mills:
Nearby businesses have been unable to gain access to their units on the site due to the large police cordon.
"The police came in the night - they have closed the whole area," said a worker at a nearby cafe, who declined to give his name.
Dan Peters, managing director of Pirtek, a hydraulic maintenance company, said: "We're not able to gain access to our units to open up for daily business.
"Obviously, it's terrible news."
Mr Peters said the industrial estate is made up of courier and logistics companies.
Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association, said the route used was "unorthodox" since it apparently involved traveling to the island of Ireland and then entering Britain via a ferry to a major passenger port in Wales.
He said the choice may have been influenced by increased security and checks in the major English port of Dover and the French port of Calais.
"People have been saying that security and checks have been increased at places like Dover and Calais, so it might be seen as an easier way to get in by going from Cherbourg or Roscoff, over to Rosslare, then up the road to Dublin," he said.
"It's a long way around and it'll add an extra day to the journey."
Richard Burnett, chief executive of the Road Haulage Society, said "this tragedy it highlights the danger of migrant gangs people-smuggling on lorries".
He said: "It's highly unlikely that if this vehicle has come from Europe that it's been physically checked.
"Because of the migrant issue at Dover and Calais, you've got far more checks."
Mr Burnett said the container appeared to be a refrigerated unit and described conditions for anyone inside as "absolutely horrendous".
Temperatures in such units can be as low as -25C if frozen products are being transported, causing humans to "lose their lives pretty quickly", he said.
He added: "It's going to be dark. If the fridge is running it's going to be incredibly cold.
"The only place to go to the toilet is on board the back of the trailer. You can imagine if they've been in there for days then there will be faeces, there will be urine."
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