Twenty-seven people have died near Calais while trying to cross the Channel, France's interior minister has said.
Gerald Darmanin, speaking from the northern French town, also said five women and a girl were among the victims.
Two people were saved from the water and four suspected people-smugglers have been arrested, he added.
Mr Darmanin had earlier said 31 people had died, but the number was later revised down.
The deaths occurred after an inflatable dinghy capsized near Calais this afternoon, with fishermen reporting more than a dozen bodies motionless in the sea.
Mr Darmanin described the boat as "very frail" and "like a pool you blow up in your garden", according to a translation.
One UK patrol boat, one French lifeboat, and three helicopters have been involved in search and rescue efforts, which were continuing this evening.
Prime Minister Boris Johnson chaired a meeting of the UK's emergency COBRA committee in response and held an urgent phone call with French President Emmanuel Macron.
Mr Johnson said he was "shocked, appalled and deeply saddened" and that human traffickers were "literally getting away with murder".
It is the worst-ever incident involving migrants in the Channel, according to French maritime authorities.
President Macron said: "Europe... is in mourning tonight." He promised "everything will be done to find and condemn those responsible" and that "France will not let the Channel become a graveyard".
Following the leaders' phone call, a Downing Street spokesperson said they "agreed on the urgency" of stepping up joint efforts to stop smuggling gangs and "underlined the importance of close working with neighbours in Belgium and the Netherlands as well as partners across the continent if we are to tackle the problem effectively before people reach the French coast".
Fisherman Nicolas Margolle said he had seen two small dinghies - one with people onboard and another empty.
He said another fisherman had called rescuers after seeing the empty dinghy and 15 people motionless in the water.
Conditions in the Channel were described as cool but calm, which may explain why there were a number of crossings.
Other migrants were brought ashore at Dover and Dungeness earlier in the day.
Mr Darmanin told reporters that 255 had made it across the Channel and 671 were stopped.
He also said 580 police had been patrolling the shore.
The Dover Strait is the world's busiest shipping lane and more than 25,700 people have made the dangerous journey to the UK this year.
That's three times the total for 2020, according to data compiled by PA news agency.
The migrant crisis has become an increasingly tense subject for the UK and France.
The government has accused the French of not doing enough to stop people, despite giving them millions in extra funding to deal with the problem.
Home Secretary Priti Patel said "the tragedy was the starkest possible reminder of the dangers of these Channel crossings organised by ruthless criminal gangs".
In a statement, she said the government's new immigration plan would "address many of the long-standing pull factors encouraging migrants to make the perilous journey".
"We will continue to intensify all cooperation with France and other European partners to prevent migrants embarking on these deadly journeys," she added.
In a statement, President Macron has called for an emergency meeting of European ministers and an "immediate strengthening" of Frontex, the EU's border agency.
He also said France had been working with the UK "for several months" to fight people smugglers and that 1,552 had been arrested.
Despite this, he said 47,000 attempted crossings had taken place and 7,800 migrants rescued since since 1 January.