British police found the bodies of 39 people inside a truck believed to have come from Bulgaria at an industrial estate to the east of London on Wednesday, and said they had arrested the driver on suspicion of murder.

The discovery was made in the early hours after emergency services were alerted to people in the truck container, on a gritty industrial site in Grays, about 20 miles from 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.

“I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened,” Johnson said on Twitter. “My thoughts are with all those who lost their lives & their loved ones.”

All those in the container, 38 adults and one teenager, 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 it could not confirm at this stage whether the truck had started its journey from the country.

“We are still checking the information, published in the British media and we're contacting the authorities,” foreign ministry spokeswoman Tsvetana Krasteva said.

Police officers in forensic suits were on Wednesday inspecting a large white container on a red truck next to warehouses at the site. Police had sealed off the surrounding area of the industrial 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, British customs officials found the bodies of 58 Chinese people crammed into a tomato truck at the southern port of Dover.