The discovery of the bodies - 38 adults and one teenager - was made in the early hours after emergency services were alerted to people in a truck container on a gritty industrial site in Grays, about 20 miles (32 km) east of central London.

The truck was thought to have entered Britain at Holyhead, a North Wales port that is a major entry point for traffic from Ireland, on Saturday and to have originally started its journey in Bulgaria, police said. The driver of the truck, a 25-year-old man from Northern Ireland, was in custody.

Prime Minister Boris Johnson said he was appalled by the news and was receiving regular updates about the investigation.

"We know that this trade is going on - all such traders in human beings should be hunted down and brought to justice," he said.

All those in the container were pronounced dead at the scene after the emergency services were called to the Waterglade Industrial Park, not far from docks on the River Thames.

Bulgaria's foreign ministry said had been in contact with the British authorities over the incident.

"At present, it has not yet been confirmed whether the truck has a Bulgarian registration," a foreign ministry spokeswoman said. "There is also no indication of the nationality of the human bodies found in the truck. British police have warned that the identification of the bodies will take a long time."

Desperate and Dangerous Situation

Prime Minister Leo Varadkar said Irish authorities would carry out any investigations necessary if it was established that the truck had passed through Ireland.

Police officers in forensic suits were inspecting a large white container on a red truck next to warehouses at the site. Police had sealed off the surrounding area of the estate with large green barriers as they carried out their investigation.

"At this stage, we have not identified where the victims are from or their identities and we anticipate this could be a lengthy process," Essex Police Deputy Chief Constable Pippa Mills told reporters. "This is an absolute tragedy."

Mills said finding out who the victims were was their top priority, while a key line of inquiry was determining the truck's route from Bulgaria to Ireland and then onto Britain.

Nearby businesses said they had 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.

For years, illegal immigrants have attempted to reach Britain stowed away in the back of trucks, often seeking to reach the United Kingdom from the European mainland.

In Britain's biggest illegal immigrant tragedy in 2000, customs officials found the bodies of 58 Chinese people crammed into a tomato truck at the southern port of Dover.

Jeremy Corbyn, the leader of the opposition Labour Party, said the latest deaths were an unbelievable human tragedy that needed answers.

"Can we just think for a moment of what it must have been like for those 39 people, obviously in a desperate and dangerous situation, for their lives to end, suffocated to death in a container," he said.

REUTERS