Thirty-one migrants 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.
Four people had been arrested near to the Belgian border, he added, saying: “We suspect that they were directly linked to this particular crossing.”
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.
Home Secretary Priti Patel tweeted that the deaths were “starkest possible reminder” of the dangers migrants face attempting to cross the Channel.
French prime minister Jean Castex said the shipwreck was a “tragedy” and those who died were victims of “criminal smugglers”.
“My thoughts are with the many missing and injured, victims of criminal smugglers who exploit their distress and injury,” he said.
French President Emmanuel Macron said: “France will not let the Channel become a cemetery.”
He promised to “find and condemn those responsible” for the tragedy.
Mr Macron also called for an “emergency meeting of European ministers concerned by the migration challenge”.
Jean-Marc Puissesseau, president and chairman of the ports of Calais and Boulogne, told BBC News 50 people had been on the boat, and about “five or six” people had been found.
A number of people are believed to have reached the UK in small boats on Wednesday, with people seen being brought ashore in Dover by immigration officials.
BBC Newsnight’s policy editor, Lewis Goodall, said he understood about 25 boats had attempted the crossing so far during the day.
It comes amid record numbers of migrants making the crossing from France to the UK. More than 25,700 people have made the dangerous journey to the UK in small boats this year – more than three times the 2020 total.
The Dover Strait is the busiest shipping lane in the world and has claimed many lives of people trying to cross to Britain in inflatable dinghies.
It is thought at least 10 other people had died in the past few weeks while attempting to make the crossing.
Mr Johnson admitted efforts so far to stem the flow of migrants crossing the Channel in small boats “haven’t been enough”.
He said the UK would offer to increase its support to France.
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.