Police were called by the ambulance service shortly before 01:40 BST after the discovery at Waterglade Industrial Park in Eastern Avenue, Grays.

The lorry driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder.

Essex Police said the vehicle travelled from Bulgaria and entered the UK via Holyhead, Anglesey, on Saturday.

Early indications are that 38 adults and one teenager have died, the force said. There were no survivors.

Deputy Chief Constable Pippa Mills said identifying the victims "remains our number one priority".

She appealed for anyone with information about the route the lorry may have taken, or anyone who had seen the vehicle or knew about its journey, to get in touch.

Prime Minister Boris Johnson told Prime Minister's Questions it was an "unimaginable tragedy and truly heartbreaking".

He said: "I know that the thoughts and prayers of all members will be with those who lost their lives and their loved ones. I'm receiving regular updates. The Home Office will work closely with Essex Police as we establish exactly what has happened."

Irish Premier Leo Varadkar told the Dail that the deaths were a "human tragedy", adding that investigations would be undertaken to establish if the lorry had passed through Ireland.

Home Secretary Priti Patel said she was "shocked and saddened by this utterly tragic incident", while Thurrock MP Jackie Doyle-Price said it was "sickening news".

"People trafficking is a vile and dangerous business," Ms Doyle-Price tweeted, adding that she hoped Essex Police could "bring these murderers to justice".

Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association, said if the lorry had come from Bulgaria, getting into Britain via Holyhead was an "unorthodox route".

He said: "People have been saying that security and checks have been increased at places like Dover and Calais, so it might be seen as an easier way to get in by going from Cherbourg or Roscoff, over to Rosslare, then up the road to Dublin.

"It's a long way around and it'll add an extra day to the journey."

The National Crime Agency said it had sent officers to assist the investigation and identify any "organised crime groups who may have played a part". Ch Supt Andrew Mariner, from Essex Police, said he expected identifying the victims would be a "lengthy process".

At the scene:

BBC Essex reporter Peter Walker

It's quite chaotic here and the number of bystanders and reporters is continuing to grow.

The industrial estate is huge and has lots of multi-national companies. The nearest residential property must be about half a mile away.

Glen Freeland, from GSF Car Parts, which is close to where the bodies were found, said staff could not access their premises on Eastern Avenue.

"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.

In June 2000, 58 Chinese immigrants were found suffocated to death in the back of a lorry in Dover. There were two survivors.

A Dutch lorry driver was jailed the following year for manslaughter.