Belgian authorities on Wednesday rescued 49 mainly Vietnamese migrants who were attempting a sea crossing to Britain, apparently from France, the Bruges prosecutor's office said.
"They were in good health but some had mild symptoms of hypothermia," the statement said, explaining that the boat had taken on water and was "in distress" when it was spotted.
A helicopter was launched to track the boat while rescuers headed to the scene and the migrants were taken ashore and transferred to the maritime police in Zeebrugge.
Police will launch an investigation "into the smuggling organisation responsible for these events."
"Based on initial findings, it is suspected that the boat departed from the French coast, but the investigation will have to confirm this," the prosecutor's office said.
Vietnamese migrants are among those camped out at Grande-Synthe on the northern French coast, trying to find a way to cross to Britain.
Around 100 Vietnamese were taken out to the camp to be rehoused in shelters on May 6, according to the Grande-Synthe town hall.
The migrants are brought to the Channel coast from their southeast Asian homeland by a network that flies them to Moscow and then transports them through Europe.
Yann Manzi from the NGO Utopia, which helps migrants, says that until recently the Vietnamese kept themselves apart from other migrant populations.
But they are now increasingly mixed up with the other African and Asian travellers waiting for a place on a boat or a chance to jump in the back of a truck.
The last leg of the trip, across the sea to southern England on a dinghy or in a truck carried by a ferry or the Channel tunnel can be the most dangerous.
In October 2019, the lifeless bodies of 39 Vietnamese migrants were discovered suffocated in a trailer in Grays Industrial Estate, east London.
The investigation established that the migrants had been picked up in northern France and had transited through the Belgian port of Zeebrugge.
Investigations were launched in France, Belgium and the United Kingdom, where a trial was held in 2020 resulting in the conviction of two key figures in the trafficking.