At least 27 people headed for the UK have drowned in the English Channel near Calais after their boat sank.
The International Organization for Migration said it was the biggest single loss of life in the Channel since it began collecting data in 2014.
Prime Minister Boris Johnson said he was "appalled" by what happened, adding the UK would leave "no stone unturned" to stop human trafficking gangs.
Five women and a girl were among the dead, France's interior minister said.
Gerald Darmanin also said two people were rescued and one was missing. It was earlier reported 31 people had died, but the total was revised down overnight on Thursday.
Four people had been arrested near to the Belgian border, he added, saying: "We suspect that they were directly linked to this particular crossing."
On Wednesday evening, Mr Johnson and French President Emmanuel Macron agreed to step up joint efforts to prevent the crossings and stop the gangs putting people's lives at risk, Downing Street said.
A fishing boat sounded the alarm on Wednesday afternoon after spotting several people at sea off the coast of France.
French and British authorities are conducting a rescue operation by air and sea to see if they can find anyone. (BBC News)
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