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, County Armagh, previously admitted conspiracy to assist unlawful immigration.
At the same Old Bailey hearing, co-defendant Gheorghe Nica, 43, denied 39 counts of manslaughter.
Both men appeared via video-link at the hearing, which was conducted virtually with most lawyers and court reporters attending by Skype.
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.
Another three men charged with other offences in connection with the deaths also appeared at the Old Bailey via video-link.
Romanian national Alexandru-Ovidiu Hanga, 27, of Hobart Road in Tilbury, Essex, denied a charge of conspiracy to assist unlawful immigration.
Christopher Kennedy, 23, of Corkley Road in Darkley, County Armagh, has previously denied conspiracy to assist unlawful immigration.
Valentin Calota, 37, of Cossington Road in Birmingham, was not asked to enter a plea to the charge of conspiring to assist unlawful immigration.
Mr Nica, a British Romanian citizen of Mimosa Close in Langdon Hills, Essex, also denied one count of conspiracy to assist unlawful immigration between 1 May 2018 and 24 October 2019.
Robinson also admitted one charge of transferring criminal property, but denied a charge of transferring criminal property.
Prosecutor William Emlyn Jones QC asked for three weeks to decide whether to proceed with a trial against Robinson on that charge.
The other defendants face a trial at the Old Bailey lasting up to eight weeks. It is scheduled to begin on 5 October.