A group of 11 Albanian migrants, including a pregnant woman and a 17-year-old boy, have been found onboard a cargo ship at Southampton Docks.
The group was discovered by port workers in Dock Gate 4 on Wednesday morning, Border Force said.
A Home Office spokeswoman said they were understood to have arrived in the city on a vessel from Le Havre, France.
Hampshire Police said the pregnant woman had been taken to hospital and the other 10 people remain in custody.
The force said the 17-year-old boy would be transferred into the care of children's services.
Associated British Ports said the stowaways were found on the vessel MV Salome.
The port operator said it was supporting "government agencies with their investigations and is working with the shipping company and owners as required".
'Third party assistance'
The ship, a vehicle carrier which can transport up to 6,000 cars at a time, is owned by shipping firm Wallenius Wilhelmsen.
In a statement the firm said: "Having gained illegal access to the Le Havre terminal, the stowaways appear to have had third party assistance in locking themselves in the loading compartment of a truck being transported on one of the cargo decks.
"Wallenius Wilhelmsen is undertaking a full enquiry in conjunction with the port authority in Le Havre as to how the stowaways gained access to the port and vessel and how the various stowaway checks failed to find evidence of illegal entry."
The firm said a full stowaway dog search was carried out in Southampton before the vessel departed this morning. The ship is currently travelling to Baltimore.
On Sunday, seven stowaways were detained after military forces stormed an oil tanker off the Isle of Wight.
The Nigerian men were found on board the Southampton-bound Nave Andromeda which had travelled from Lagos.
The Special Boat Service ended a 10-hour standoff which started when the stowaways reportedly became violent.
All seven remain in custody on suspicion of seizing or exercising control of a ship by use of threats or force under the Aviation and Maritime and Security Act 1990, police said.