British police say the bodies of 39 people have been found in a truck container at an industrial estate to the east of London.
Police said the truck was believed to have come from Bulgaria and to have entered Britain at Holyhead in Wales on Saturday.
The driver, a 25-year-old man from Northern Ireland, has been arrested.
"This is a tragic incident where a large number of people have lost their lives," Chief Superintendent Andrew Mariner said.
"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."
The bodies were discovered in the container at Waterglade Industrial Park in Grays, near Essex located by the River Thames, about 32 kilometres from central London.
Police said all 39 victims — 38 adults and one teenager — were pronounced dead at the scene after a local ambulance crew attended in the early hours of the morning.
British Prime Minister Boris Johnson said he was "appalled by this tragic incident".
"I am receiving regular updates from the Home Office and will work closely with Essex Police as we establish exactly what has happened," he said.
"My thoughts are with all those who lost their lives and their loved ones," the Prime Minister added.
Other UK politicians and MPs were quick to express their shock on social media. Home Secretary Priti Patel led the way.
Police had set up a perimeter around the site but local businesses told the BBC they were affected.
Glen Freeland, from GSF Car Parts, which is close to where the bodies were found, said staff could not access their premises.
"The manager went to get into work this morning and it was cordoned off and we've been moved to a different area," Mr Freeland said.
Local Thurrock MP Jackie Doyle-Price blamed people smugglers for the deaths.
"People trafficking is a vile and dangerous business … let's hope they bring these murderers to justice," she tweeted.
In June 2000, the bodies of 58 Chinese immigrants were found in the back of a truck in the sea port of Dover. The Dutch driver was jailed the following year for their manslaughter.
