The driver of a lorry in which 39 people died called 999 and described finding "loads" of migrants "jammed" into his trailer, a court heard.
The Vietnamese men, women and children were found dead in a container on an industrial estate in Purfleet in Essex on 23 October 2019.
Maurice Robinson is said to have waited 23 minutes before calling for help.
The 26-year-old has already pleaded guilty to manslaughter while two men are on trial for the same charge.
One of the men, and a further two men, face people smuggling charges.
The court was shown CCTV of Robinson collecting the trailer, which had been transported from Zeebrugge in Belgium, and driving out of Purfleet port.
After discovering the bodies, prosecutors said, he made several phone calls to others accused of roles in the people-trafficking scheme and drove the lorry in a loop around the industrial area, before calling the emergency services 23 minutes later.
During a call to the ambulance service, he said none of the migrants was breathing.
"No. There, there's, err, loads of them, there's immigrants in the back but they're, they're all lying on the ground," he said.
"I went and lifted a trailer from Purfleet, the freight terminal, and I got around to where I was gonna park up for the night and I heard a noise in the back and I opened the door and there's a bunch of them lying."
Asked how many people were involved, Robinson, of Craigavon, County Armagh, said: "The trailer is jammed. I don't know."
Jurors have heard the victims suffocated inside the sealed trailer as the temperature reached 38.5C (101F).
Earlier the court heard how one of the people in the container was recorded on a mobile phone saying "I cannot breathe".
Nguyen Dinh Luong, 20, was also recorded saying: "I'm sorry. It's all my fault." A voice in the background then says: "He's dead."
Cargo operator Jason Rook said in a statement he noticed a "decomposing smell" as he unloaded the trailer at Purfleet.
He said the ship carrying it arrived at about 23:50.
He said: "As I just passed the doors and reached the left hand side of the trailer I suddenly caught a strong smell that I can only describe as a decomposing smell."
Between 22:00 and 22:30 it was believed that carbon dioxide in the trailer reached the "toxic threshold", the court heard.
Jurors heard how the trailer was loaded onto the Clementine at the port of Zeebrugge at 15:00 on 22 October last year.
Gheorghe Nica, 43, of Basildon, Essex, and lorry driver Eamonn Harrison, 23, deny the manslaughters of 39 Vietnamese people, aged between 15 and 44.
Mr Harrison, of Mayobridge, County Down, Christopher Kennedy, 24, of County Armagh, and Valentin Calota, 37, of Birmingham, deny being part of a people-smuggling conspiracy, which Mr Nica has admitted.
The trial continues.