Four Iraqis were found in the back of a refrigerated lorry bound for Britain in temperatures of minus 20C (minus 4F), an inspection report has revealed.
The group was discovered on the approach to the Channel Tunnel at Coquelles in northern France in July.
They were taken from the vehicle by UK Border Force officers before being transferred to French authorities.
The episode was revealed in a report on the short-term holding facilities which form part of Britain's "juxtaposed" border controls in France.
Inspectors were critical of the state of two of the centres where detainees are taken - including one which was still in use despite the Home Office facing calls to close or refurbish it four years ago.
The Coquelles freight facility is used to hold those found hidden in commercial vehicles in freight lanes.
Most detainees were found hidden in the back of lorries waiting in freight lanes to board trains passing through the Channel Tunnel, the report from HM Inspectorate of Prisons said.
It added: "During our inspection, four young Iraqis were found in the rear of a refrigerated lorry in a temperature of about minus 20 degrees centigrade.
"Border Force got the young men out of the lorry quickly and wrapped them in foil blankets."
Later the same day, a family of three adults and four children were found hidden in a different lorry.
The Coquelles freight facility was still in use and unchanged - despite a recommendation that it be closed or completely refurbished at the last inspection in November 2012, according to the report.
It said: "The accommodation was poor. Detainees were held in stark, poorly ventilated holding rooms with squat toilets in the corner."
Since the inspection a new facility has been opened at the Coquelles freight site.
A second facility at Coquelles is used to hold people travelling on coaches and cars.
Some detainees tried to enter the UK clandestinely in vehicles; others presented themselves at border control in good faith but did not meet the immigration rules, according to the report.
It said the facility remained "uncomfortable and noisy" with no shower or adequate sleeping facilities.
Another centre at Calais had been rebuilt and improved since it was last inspected, HMIP added.
A separate report from the watchdog examined holding facilities at Kent.
It found that the number of detainees passing through the Dover Seaport holding room had fallen significantly since last year, with 967 recorded in the three months to July 31 2016, compared with 2,781 in the three months to September in 2015.
Another site, Longport freight shed, was no longer being used while a third, an overflow facility, had not been used in almost a year.
"These operational changes reflected a significant decrease in the number of clandestine migrants seeking entry into the UK, via the Channel Tunnel in particular," the report said.
Britain has committed around £85 million in total to reinforce security in the Calais region.
In one project, a barrier dubbed the "great wall of Calais" - a 13ft (4m) high concrete structure - is being built along the main motorway to the port.