At least four people died when a small boat apparently packed with migrants capsized in freezing temperatures in the Channel overnight, the UK government said on Wednesday.
Dozens of others were plucked from the waters of one of the world's busiest shipping lanes in a large-scale rescue operation involving UK and French emergency services.
UK Prime Minister Rishi Sunak called it a "tragic loss of human life", just as he tries to tighten rules to prevent record numbers of migrants from attempting the crossing.
British media said earlier that 43 people were rescued, including more than 30 who had fallen overboard, with fears the death toll will rise.
Migrants have been intercepted regularly in the Channel in recent years, using small boats ill-suited for trips on the open sea.
At least 27 people drowned while attempting to cross the Channel in a dinghy on November 24 last year.
The International Organization for Migration estimates that 205 migrants have been recorded as missing in the Channel since 2014.
Nikolai Posner, from the Utopia 56 group helping migrants in northern France, said they received a voice message and location notification from a boat in distress at 2:53 am local time (0153 GMT).
"We forwarded it to the French and British coastguard by phone. At 3:40 am (0240 GMT), the French coastguard told us the British were handling it," he told AFP.
"The location that was sent to us was in French waters. At 2:59 am the person who contacted us was no longer receiving messages on WhatsApp."
Posner, however, said he could not be sure if the message originated from the same small boat.
Lifeboats
The UK Maritime and Coastguard Agency (MCA) coordinated the rescue operation, which also involved Border Force, police and other emergency responders.
Lifeboats were launched from the Channel port of Dover at 3:07 am British time (0307 GMT), followed by vessels from Ramsgate and Hastings along the coast, it added.
A government spokesman said: "At 0305 today, authorities were alerted to an incident in the Channel concerning a migrant small boat in distress.
"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 are ongoing and we will provide further information in due course."
The MCA said at least four lifeboats and three coastguard rescue teams were dispatched, as well as two coastguard helicopters. A fishing vessel in the area also helped.
French officials provided a helicopters and a navy patrol boat.
Tens of thousands of migrants now regularly attempt to cross the Channel from northern France to southern England in small boats, in a trend that has grown hugely in recent years.
More than 43,000 migrants have made the journey across the Channel so far this year -- a record -- creating tensions between London and Paris about preventative measures.
Wednesday's incident came the day after UK Prime Minister Rishi Sunak announced a new deal with Albania to stem the flow of migrants crossing the Channel from mainland Europe.
A third of all those arriving in UK waters this year -- almost 13,000 -- have been Albanian.
He said that, under the agreement, Albanians arriving by boat across the Channel would be immediately returned to their home country.
Freezing weather conditions in northern Europe and windy conditions on the Channel have deterred crossings in recent days.
But a drop in the wind appears to have prompted the latest attempt.
Migrant welfare charities operating in France said winter conditions posed added to the dangers of trying to cross the Channel illicitly.
"Crossings are even more difficult in winter," said Utopia 56's Posner.
"The cold makes a difference if people fall overboard, the survival time in the water is much lower."
He added the risk of deadly hypothermia was "extremely high" once people entered the water.
