Lorry driver Maurice Robinson has pleaded guilty at the Old Bailey to 39 counts of manslaughter after the deaths of a group of Vietnamese migrants.
The 31 males and eight females were discovered in a refrigerated trailer on 23 October in Grays, Essex.
Robinson, 25, of Craigavon, Northern Ireland, previously admitted conspiracy to assist unlawful immigration.
At the same Old Bailey hearing, co-defendant Gheorghe Nica denied 39 counts of manslaughter.
Both men appeared via video-link.
The bodies of the Vietnamese nationals were discovered at an industrial estate soon after the lorry arrived in the UK on a ferry from Zeebrugge in Belgium.
Among the men, women and children were 10 teenagers, two of them 15-year-old boys.
In March, it was revealed they all died from asphyxia and hyperthermia.