Police forensic officers at the scene after a truck was found with dead bodies. AP

British police have confirmed that 39 people found dead in a container truck near an English port were Chinese citizens.

The Essex Police force says the dead found Wednesday in the southeastern city of Grays included 31 men and eight women. The force says one victim previously thought to be a teenager was a young adult woman.

The 25-year-old truck driver, who is from Northern Ireland, is being questioned on suspicion of attempted murder but has not been charged. Police in Northern Ireland have searched three properties there as they try to reconstruct the path of the truck and the victims' final journey.

This photo shows a truck seen in rear in Thurrock, England, on Wednesday. AP

"I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened," Johnson said on Twitter. "My thoughts are with all those who lost their lives & their loved ones."

According to Police, the truck got into Britain through Holyhead, a North Wales port that serves as a major entry point for traffic from Ireland. It is believed that the truck started its journey from Bulgaria.

Bulgaria's foreign ministry said it could not confirm at this stage whether the truck had started its journey from the country.

"We are still checking the information, published in the British media and we're contacting the authorities," foreign ministry spokeswoman Tsvetana Krasteva said.

Police officers block the road near the scene where a truck was found with dead bodies. AP

"This is a tragic incident where a large number of people have lost their lives. We are in the process of identifying the victims, however I anticipate that this could be a lengthy process," said Chief Superintendent Andrew Mariner, from Essex Police.

"We have arrested the lorry driver in connection with the incident who remains in police custody as our enquiries continue."

Nearby businesses said they had been unable to gain access to their units on the site due to the large police cordon.

"The police came in the night — they have closed the whole area," said a worker at a nearby cafe, who declined to give his name.

Four years ago, four children, including a baby girl, were among 71 people found dead in a truck on an Austrian road. The lorry consisted largely of immigrants.

The refugees appeared to have been dead for nearly two days. A Syrian travel document was found among the victims .

The deaths threw light on the risks faced by migrants at the hands of traffickers on arrival in Europe.

In another major immigrant tragedy in 2000, British customs officials found the bodies of 58 Chinese people crammed into a tomato truck at the southern port of Dover.