Officers inspected the lorry at the Waterglade Industrial Park in Grays. Photo: PA Media

The vehicle was found shortly before 01:40 BST at Waterglade Industrial Park in Grays, police said.

Essex Police said the trailer has been moved to a secure location where the bodies of those inside - a teenager and 38 adults - can be recovered.

The driver, named locally as Mo Robinson, 25, from Portadown, County Armagh, Northern Ireland, has been arrested on suspicion of murder.

Essex Police said the trailer arrived via ferry from Zeebrugge, Belgium, into Purfleet on the River Thames.

The ship docked in the Thurrock area shortly after 00:30, the force said.

It is believed the container and lorry then left the port together about 35 minutes later.

Deputy Chief Constable Pippa Mills said identifying the victims remained a "number one priority", but was expected to be a "lengthy process".

The National Crime Agency said it had sent officers to assist and identify any "organised crime groups who may have played a part".

Richard Burnett, chief executive of the Road Haulage Association, said the container appeared to be a refrigerated unit, where temperatures could be as low as -25C.

He described conditions for anyone inside as "absolutely horrendous".

A spokesman for the Bulgarian foreign affairs ministry confirmed the truck was registered in the country.

"The Scania truck was registered in Varna (on the east coast) under the name of a company owned by an Irish citizen," he said.

"Police said that it is highly unlikely that they are Bulgarians," he added.

Police have appealed for witnesses and anyone with information about the lorry's route to contact them.

The force said it believed the tractor unit - or front part - of the lorry had come from Northern Ireland.

Prime Minister Boris Johnson said it was an "unimaginable tragedy and truly heartbreaking".

Speaking at Prime Minister's Questions, he said: "I know that the thoughts and prayers of all members will be with those who lost their lives and their loved ones.

"I'm receiving regular updates. The Home Office will work closely with Essex Police as we establish exactly what has happened."