LONDON: A British lorry driver who discovered dozens of dead Vietnamese migrants in a container waited 23 minutes before alerting police, a court was told on Tuesday.

Maurice Robinson, 26, who has pleaded guilty to the manslaughter of the 39 migrants and to conspiring in people-smuggling, made the grim find in the early hours of October 23 last year.

He had collected the sealed container from the southeast English port of Purfleet after it had arrived on a cargo ship from Zeebrugge in Belgium, and opened its doors shortly afterwards in nearby Thurrock.

Inside he found the bodies of the migrants -- including two boys aged 15 -- who had suffocated to death after being trapped in the dark inside for at least 12 hours, in unbearably high temperatures.

However, instead of immediately calling police Robinson made several phone calls to others accused of roles in the people-trafficking scheme, a prosecutor told London's Old Bailey court.

He also drove the lorry in a loop around the surrounding industrial area, taking 23 minutes before alerting officers, jurors in the trial of four men charged with involvement heard.

Eamonn Harrison, 23, who is said to have driven the lorry to Zeebrugge, and Georghe Nica, 43, both deny 39 counts of manslaughter.

Harrison, Valentin Calota, 37, and Christopher Kennedy, 24, have pleaded not guilty to being part of a people-smuggling conspiracy. But Nica has admitted that charge.

The trial, which began earlier this month, is expected to last up to six weeks.

Prosecutors have previously outlined that haulage company boss Ronan Hughes instructed Robinson via a Snapchat message to "give them air quickly but don't let them out" once he picked up the container in Purfleet.