The truck, which is said to have entered Britain via the Welsh port of Holyhead on Saturday, was found by ambulance workers at Waterglade Industrial Park in Grays, a town just by the River Thames around 25 miles (40 kms) east of central London.
“This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened,” Essex Police Chief Superintendent Andrew Mariner said.
“We are in the process of identifying the victims, however I anticipate that this could be a lengthy process.”
A 25-year-old-man from Northern Ireland who was driving the truck has been arrested on suspicion of murder. He remains in custody. He has not been charged or identified.
A cordon has been put in place and access to and from the industrial park remains closed.
“We are working with Thurrock Council (the local authority) to mitigate against any impact our investigation scene will have locally.” Mariner said.
Prime Minister Boris Johnson pledged in a tweet to work closely with Essex Police to establish exactly what happened.
“My thoughts are with all those who lost their lives & their loved ones,” he said.
Police have not formally linked the deaths to people trafficking but a link is assumed because of the way the victims were crammed into the truck container.
The tragedy recalls the death of 58 migrants in 2000 in a truck in Dover and the deaths in 2015 of 71 migrants from Syria, Iraq and Afghanistan found suffocated in the back of a refrigerated truck abandoned in the emergency lane of a highway near Parndorf, Austria, close to the Hungarian border.
Smaller numbers of migrants have occasionally been found dead in trucks in Britain in recent years.
Bulgarian authorities said they could not yet confirm that the truck had started its journey from Bulgaria.
“We are in contact with our embassy in London and with British authorities,” foreign ministry spokeswoman Tsvetana Krasteva said.
