LONDON (Reuters) - British police found 39 bodies on Wednesday in a truck container near London believed to have come from Bulgaria, while a 25-year-old Northern Ireland driver was arrested, police said.

The British Prime Minister Boris Johnson, shocked by the incident, Johnson wrote on the social networking site "Twitter": "The tragic incident shocked me."

The victims were declared dead at an industrial complex in the city of Griez, east of London, according to a statement from Essex police.

"This is a tragic incident in which many people have lost their lives," Essex police chief Andrew Mariner said in a statement, adding that investigations were underway to find out what had happened.

Police said the ambulance service informed police around 0040 GMT, after the discovery of a container truck found inside people in the Waterglad industrial complex.

Mariner said: "We are in the process of identifying the victims. But I expect to take a long time, we believe that the truck came from Bulgaria, we stopped the truck driver on the back of the incident and investigations are continuing."