Fifty people were rescued after a second boat containing migrants started sinking on the same night four people died in a separate incident in the English Channel.
Five people out of the 50 had ended up in the freezing water, according to government officials.
The migrants were saved by Border Force Officers in what has been a tragic week in the world’s busiest shipping lane.
Kent Police and the National Crime Agency are looking into the circumstances surrounding the separate incident early on Wednesday where four lives were lost and 39 other people were rescued from a capsized dinghy.
Officers are trying to identify the victims and trace their relatives after the search for others who may have been caught up in the tragedy was called off at 5pm on Thursday.
A teenager was among those who perished and at least a dozen children were also on board the inflatable vessel.
A British fisherman whose crew saved 31 people told Sky News he was woken in the early hours of Wednesday morning when migrants surrounded his boat “screaming for help” – and a French charity said they received a mayday call.
He said those he rescued came from Afghanistan, Iraq, Senegal and India, and told him they had each paid £5,000 to a smuggler in France for passage into the UK.
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On Thursday, the Ministry of Defence (MoD) said 401 migrants were detected in eight boats the day before, not including the four who died.
The latest figures take the provisional total for the number of migrants who have made the crossing so far this year to 45,223.
This compares with more than 28,000 who crossed in small boats last year.
Earlier this week, Prime Minister Rishi Sunak announced more funding for the NCA – Britain’s equivalent of the FBI – to tackle organised immigration crime in Europe.
‘We are determined to find those responsible’
NCA director-general Graeme Biggar said of the deadly capsizing on Wednesday: “This incident, tragically, highlights the dangers of these crossings, a high percentage of which are facilitated by organised criminal networks.
“They treat people as a commodity to be profited from and think nothing of putting them in incredibly dangerous situations. Working with our partners on both sides of the Channel we are determined to find those responsible and bring them to justice.”
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The NCA is also taking a role in the French investigation into the deaths of at least 27 migrants in the Channel last year.
Campaigners have organised an event in Folkestone on Saturday to call on the government to establish safe routes for asylum seekers to claim asylum in the UK.
Organiser Bridget Chapman said people “sick of this avoidable and unnecessary waste of life” will meet on Sunny Sands beach.