Dozens were also rescued after an overnight emergency operation off the coast of Kent in southern England, where many small boats arrive carrying migrants from France.
Four people have died after a small boat carrying migrants capsized in freezing waters in the English Channel during the early hours on Wednesday morning.
More than 40 people were rescued after lifeboats, helicopters and rescue teams working with the French and British navies responded to the emergency concerning what the UK government described as "a small migrant boat in distress".
It comes as immigration to Britain organised by people-smuggling criminal gangs has become a priority issue for Prime Minister Rishi Sunak's government.
"At 0305 (GMT) today, authorities were alerted to an incident in the Channel concerning a migrant small boat in distress," a British government spokesperson said in a statement.
"After a coordinated search and rescue operation led by HM Coastguard, it is with regret that there have been four confirmed deaths as a result of this incident." Investigations were underway, the statement added.
UK Home Secretary Suella Braverman told parliament the tragedy was a "sobering reminder" of why dangerous journeys across the Channel had to end.
Earlier on Wednesday she said in a tweet that she was "aware of a distressing incident in the Channel" and was waiting for more information. "My heartfelt thoughts are with all those involved," she added.
Record numbers of migrants have been using small boats in hopes of crossing the Channel to claim asylum in the UK, putting the government under pressure to curb the flow.
The rising numbers are thought to be the result of increased activity by people-smuggling gangs, reinforced security around regular Channel crossing routes, and -- government critics say -- the lack of legal paths for people to come to the country.
The disaster happened in freezing conditions off the coast of Kent, where many small boats arrive carrying migrants from France.
LBC radio station reported that 43 people had been rescued. A Reuters journalist saw one body bag being removed from a vessel at the lifeboat station in the port of Dover.
Sunak pledges more resources, and deterrents
Rishi Sunak pledged on Tuesday to clear a huge backlog of asylum-seeker applications with more resources, and announced new measures aimed at curbing the number of migrants reaching the UK by crossing the Channel.
The prime minister said he planned to introduce legislation early next year to ensure people who arrive illegally cannot remain in the country.
The extra staff will also focus on the swift removal of Albanian migrants who have arrived via the Channel in increasing numbers, Sunak said.
Last month the UK and France pledged to ramp up efforts to stop migrants and refugees from making perilous journeys across the English Channel, by stepping up patrols, embedding British officials with French counterparts, and boosting security equipment.
They also vowed to create a new taskforce to reverse "the recent rise in Albanian nationals and organised crime groups exploiting illegal migration routes".
France has succeeded in preventing tens of thousands of small boat journeys in the Channel, but the task of patrolling its long northern coastline is huge.
Some in the UK's ruling Conservative Party accuse the French of doing too little, while politicians in France have blamed lax labour market regulations in Britain for acting like a magnet for migrants.
The latest tragedy occurred just over a year after 27 people died while attempting to cross the sea in an inflatable dinghy in November 2021, in the worst recorded accident of its kind in the Channel.
A record 44,000 people have attempted the dangerous journey across the busy waterway on small boats and made it to the UK this year, compared to 23,000 last year and 8,500 in 2020, according to government figures.
More than half of those who arrived in 2022 are said to come from Albania, Afghanistan and Iran.
UK government figures say there were 63,089 asylum applications (relating to 75,181 people) in the country in the year ending June 2022. That is well below the number of first-time applicants in France (103,800 in 2021), or Germany (148,200), according to EU statistics.