The discovery was made at 01.40 on Wednesday morning in an industrial area in Gray's east of London. The place is now closed off.
Police say to the BBC that work has begun on identifying the dead, but it may take time. According to information, it should be 38 adults and one boy in their teens.
- This is an absolute tragedy. We will work with other authorities to find out what is behind these deaths. We are at an early stage in what is likely to be a lengthy investigation, says Pippa Mills, of the Essex Police, during a press conference.
Photo: Eva Eliasson, SVT Boris Johnson: "I'm horrified"
UK Prime Minister Boris Johnson comments on the incident on Twitter.
“I am appalled by the tragic event in Essex. I get regular updates and the Home Office will be working with the Essex police to find out what has happened. My thoughts go to all those who have lost their lives and their loved ones, ”Boris Johnson writes on Twitter.
See the aerial photos from the police fences around the truck. Photo: EBU Believers come from Bulgaria
Police suspect the truck was coming from Bulgaria and entered England via Wales over the weekend. But Bulgaria's foreign ministry is yet to confirm the information.
"We are investigating the information published in the British media and we are contacting relevant authorities," a spokesman for the ministry said, according to Reuters.