At least 27 migrants trying to reach England from France died when their boat sank off the northern French coast.
It is the deadliest disaster since the Channel became a hub for such crossings.
It had earlier been reported that at least 30 people had died, however French officials later revised the number down to 27.
President Emmanuel Macron vowed France would not allow the Channel to become a "cemetery" and called for an emergency meeting of European ministers on the crisis.
"It is Europe's deepest values - humanism, respect for the dignity of each person - that are in mourning," Mr Macron said.
The disaster, the worst single loss of life since at least 2018 when migrants began using boats en masse to cross the Channel, comes as tensions grow between London and Paris over the record numbers of people crossing.
Prosecutors opened a manslaughter probe after the boat sank off the northern port city of Calais.
Interior Minister Gerald Darmanin said four suspected traffickers accused of being directly linked to the doomed crossing in a long inflatable boat had been arrested.
Mr Darmanin told reporters in Calais that only two survivors had been found and both of their lives were in danger. He said five women and one young girl were among those who died.
The nationality of the migrants was not immediately clear.
French Prime Minister Jean Castex will hold a crisis meeting early tomorrow, his office said.
Taoiseach Micheál Martin said the incident was "truly shocking".
In a social media post, Mr Martin said every effort must be made to ensure tragedies like this do not happen again.
"My thoughts go to all the victims and their families," he said.
French officials said hree helicopters and three boats had searched the area, uncovering corpses and people unconscious in the water, after a fisherman sounded the alarm.
The victims were among around 50 people aboard a vessel that had set out from Dunkirk east of Calais, according to the police.
France and UK 'to step up joint efforts'
British Prime Minister Boris Johnson and French President Emmanuel Macron have agreed to step up joint efforts to prevent migrants' crossings in the English Channel following today's tragedy.
A Downing Street spokesperson said in a statement that the two leaders "agreed on the urgency of stepping up joint efforts to prevent these deadly crossings and to do everything possible to stop the gangs responsible for putting people's lives at risk.
"They 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," the spokesperson added.
Earlier, Mr Johnson said he was "shocked, appalled and deeply saddened by the loss of life at sea", following a crisis meeting with senior officials.
But underlining the tensions between London and Paris, he also said Britain had faced "difficulties persuading some of our partners, particularly the French, to do things in a way that the situation deserves".
Britain has urged tougher action from France to stop migrants from making the voyage.
The issue has added to growing post-Brexit strains between Britain and France, with a row on fishing rights also still unresolved.
We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences
'Channel risks being as deadly as Mediterranean'
Pierre Roques of the Auberge des Migrants NGO in Calais said the Channel risked becoming as deadly as the Mediterranean, which has seen a much heavier toll over the migrants crossing.
"People are dying in the Channel, which is becoming a cemetery. And as England is right opposite, people will continue to cross."
According to the French authorities, 31,500 people attempted to leave for Britain since the start of the year and 7,800 people have been rescued at sea, figures which doubled since August.
In Britain, Mr Johnson's Conservative government is coming under intense pressure, including from its own supporters, to reduce the numbers crossing.
Natalie Elphicke, the Conservative MP for the British Channel port of Dover, called the sinking "an absolute tragedy" and demonstrated the need to stop the crossings at source.
"As winter is approaching the seas will get rougher, the water colder, the risk of even more lives tragically being lost greater," she said.
Charlotte Kwantes of Utopia56, an association that works with migrants in Calais, said "more than 300" migrants had died since 1999 in the area.
"As long as safe passages are not put in place between England and France, or as long as these people cannot be regularised in France... there will be deaths at the border," she told AFP.
According to British authorities, more than 25,000 people have now arrived illegally so far this year, already triple the figure recorded in 2020.