People are reported to have died after a small migrant boat capsized in the English Channel off the coast of Kent, near Dungeness.
A major search and rescue operation is ongoing, and between 30 to 50 people are said to have been on board the vessel when it ran into difficulties, according to a French source.
Searches will continue all day, but at this stage many are feared dead given the freezing conditions of the water, a separate source has told Sky News.
Temperatures have plunged below zero every night this week.
The Maritime and Coastguard Agency said HM Coastguard is working with the RNLI, Royal Navy, Border Force, French navy and Kent Police to look for the boat, while an air ambulance has also been sent to the scene.
Channel incident – latest updates: Helicopters and lifeboats scrambled to rescue vessel off UK coast
A spokesperson said: “HM Coastguard is currently coordinating a search and rescue response to an incident involving a small boat off Kent, working with the Navy, Border Force, Kent Police and other partners.
“We have sent Dover, Dungeness, Hastings and Ramsgate RNLI lifeboats and Deal, Dungeness and Folkestone coastguard rescue teams, along with the coastguard area commander.
“HM Coastguard helicopters from Lydd and Lee on Solent and one from the French Navy are involved. A fishing vessel in the area is also assisting in the rescue.
“South East Coast Ambulance and Kent Police are working with us and an air ambulance has been sent.”
A UK government spokesperson said: “We are aware of an incident in UK waters and all relevant agencies are supporting a co-ordinated response. Further details will be provided in due course.”
Home Secretary Suella Braverman tweeted that she is “aware of a distressing incident” in the Channel this morning and is “being kept constantly updated” while agencies respond and “urgently establish the full facts”.
“My heartfelt thoughts are with all those involved,” she added.
Dover MP Natalie Elphicke said: “I am very saddened to hear that lives are feared to have been lost following a small boat tragedy in the English Channel this morning. My thoughts and prayers are with all those involved.”
The incident comes just hours after Prime Minister Rishi Sunak promised to bring in new laws to tackle illegal immigration.
Among a raft of new measures unveiled to curb Channel crossings, he told MPs: “We have to stop the boats. And this government will do what must be done.”
As of midnight last night, 44,711 people had crossed the Channel aboard small boats in 2022, according to data analysed by Sky News.
This includes 1,087 boats, with an average of 41 people per vessel.
In 2021, there were 28,526 arrivals.