People are feared to have died after a small migrant boat ran into difficulties in the early hours in the English Channel near Kent.
The BBC's Simon Jones said it was understood there had been deaths but authorities have not confirmed them.
The home secretary said her "heartfelt thoughts" were with those involved.
The boat is likely to have been carrying migrants risking the crossing from France, a day after Prime Minister Rishi Sunak announced new measures to "stop the boats".
It is understood the small boat got into trouble off the coast of Dungeness, 30 miles west of Dover.
The UK coastguard, the French Navy 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.
