The grim discovery of 31 men and eight women inside the container truck on an industrial estate east of London last year threw fresh light on the plight of migrants desperate to reach Britain.

A post-mortem examination found the victims - 10 of them teenagers, including two 15-year-old boys - died from lack of oxygen and overheating in the refrigerated lorry.

Seven people received jail sentences in Vietnam last month for their role in the tragedy.

The four men on trial at the Old Bailey court in central London from Monday face a range of charges, from manslaughter to conspiracy to commit unlawful immigration.

They do not include the 25-year-old driver of the lorry, Maurice Robinson of Northern Ireland.

He drove the truck onto a ferry from the Belgian port of Zeebrugge in the early hours of October 23, 2019.

Robinson admitted 39 counts of manslaughter and one of conspiracy to commit unlawful immigration at an April hearing.

Meanwhile another man, 40-year-old Ronan Hughes, also from Northern Ireland, pleaded guilty to the same charges at a hearing on August 28.

At an extradition hearing in Dublin on May 15, he was described as the "ringleader" of a human trafficking operation.

Both men will be sentenced at a later date.

'I'm dying because I can't breathe'

More than two dozen other suspects were arrested in May in connection with people trafficking in France, Belgium and Germany as a result of the investigation into the case.

An alleged key figure in the ring of smugglers, a 29-year-old man nicknamed "The Bald Duke" according to AFP sources, was caught in Germany's Upper Rhine region.

Thirteen of the suspects arrested by French police have been charged there with people trafficking, while six of the group -- mainly Vietnamese and French nationals -- also face manslaughter charges.

The investigation found the migrants who died were loaded into the truck in northern France, and that the network continued to operate even after the tragedy, charging up to 20,000 euros ($23,458) to cross from France to Britain.

The family of one victim, 26-year-old Pham Thi Tra My, said they had received a text message from her in the hours before she is believed to have perished.

"I'm sorry Mom. My path to abroad doesn't succeed. Mom, I love you so much! I'm dying because I can't breathe," she said in the message confirmed by her brother Pham Manh Cuong.

The victims came from impoverished and remote corners of central Vietnam, a hotspot for people willing to embark on dangerous journeys in the hope of striking it rich abroad.

Many are smuggled illegally through Russia or China, carrying falsified documents, often owing tens of thousands of dollars to their traffickers.

They end up working off the books on cannabis farms or in nail salons.