At least 27 people headed for the UK have drowned in the English Channel near Calais after their boat sank.
UK International Organization for Migration said it was the biggest single loss of life in the Channel since it began collecting data in 2014.
The 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.
Mr Johnson said the deaths were a “disaster”, adding that it was vital to “break” the people trafficking gangs which, he said, were “literally getting away with murder”.
Speaking after chairing an emergency Cobra meeting, the prime minister said more needed to be done to stop criminals organising crossings.
“It also shows how vital it is that we now step up our efforts to break the business model of the gangsters who are sending people to sea in this way,” he said.
Downing Street later said Mr Johnson and Mr Macron agreed on the importance of close working with neighbours in Belgium and the Netherlands, as well as other European countries, to tackle the problem before people reach the French coast.
The UK has pledged to give France €62.7m (£54m) during 2021-22 to help increase police patrols along its coastline, boost aerial surveillance and increase security infrastructure at ports.
Home Secretary Priti Patel tweeted that the deaths were “starkest possible reminder” of the dangers migrants face attempting to cross the Channel.
