Four people died after a migrant boat ran into difficulties crossing the English Channel in the early hours, the government has said.
The BBC's Nick Eardley said a person close to the situation said 43 people had been saved, with more than 30 rescued from the water.
A major search and rescue operation was launched in waters between Kent and France in freezing conditions.
"These are the days that we dread," Home Secretary Suella Braverman said.
Prime Minister Rishi Sunak earlier expressed his sorrow at the "tragic loss of human life".
The boat is likely to have been carrying migrants risking the crossing from France, a day after Mr Sunak announced new measures to "stop the boats".
A government spokesman said authorities were alerted at 03:05 GMT to a small boat in difficulty off the coast of Dungeness, 30 miles west of Dover.
In a statement to the House of Commons, Ms Braverman said the search and rescue operation was ongoing, and she has spoken to Border Force officials about the tragedy.
The UK coastguard, the French Navy, the RNLI and an air ambulance were all sent to help with the rescue operation.
A fishing boat in the area and coastguard helicopters from Lydd and Lee on Solent were also involved.
South East Coast Ambulance Service said it was called following reports of the incident, and sent crews to Dover, in Kent, to help with the follow-up operation.
Overnight on Tuesday, temperatures dropped to 1C, with it likely to have been colder out at sea. A yellow weather warning for ice was in place across Kent at the time.
A number of politicians have expressed their condolences, including Home Secretary Suella Braverman, who said that her "heartfelt thoughts" were with those involved.
Labour leader Sir Keir Starmer said it was "heartbreaking" that there have been more deaths in the Channel, and Dover MP Natalie Elphicke said she was "very saddened" to hear of the tragedy.
The operation follows a fatal incident in November 2021, when at least 27 migrants died after a dinghy sank while heading to the UK from France.
The BBC's Simon Jones said 460 people made the journey from France to Kent in small boats between Friday and Sunday.
Nearly 45,000 people have made journey this year so far.
The Archbishop of Canterbury said he was "praying for the victims of today's terrible events", tweeting that debates about asylum seekers "are not about statistics, but precious human lives".
Tim Naor Hilton, from the charity Refugee Action, said the tragedy was predictable and inevitable, and more people would die trying to reach safety if the government did not create more routes for people to claim asylum.
Source: BBC
Image: Reuters
