A number of migrants headed for the UK have drowned in the Channel near Calais after their boat sank, the French government has said.
French prime minister Jean Castex said the shipwreck was a “tragedy”, adding those who died were victims of “criminal smugglers”.
Police say more than 20 people have died, AFP news agency has reported.
French and British authorities are conducting a rescue operation in the Channel by air and sea.
A fishing boat sounded the alarm earlier on Wednesday after spotting several people at sea off the coast of France.
“My thoughts are with the many missing and injured, victims of criminal smugglers who exploit their distress and injury,” Mr Castex said.
French interior minister Gerald Darmanin also confirmed on Twitter that people had died, but he did not give a number.
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A number of people are believed to have reached Britain in small boats on Wednesday, with people seen being brought ashore in Dover by immigration officials.
BBC Newsnight’s policy editor, Lewis Goodall, said he understood about 25 boats had attempted the crossing so far on Wednesday.
The French coastguard had issued a mayday distress call for 15 people who had been tipped into the water, our correspondent said.
He added that an aid agency warned there was a risk the Channel could become a “graveyard”.
It comes amid record numbers of migrants making the crossing from France to the UK. On Monday, the number of migrants to have reached the UK by boat this year surpassed more than three times the 2020 total.
More than 1,000 migrants arrived in one day earlier this month – a new record.
The Dover Strait is the busiest shipping lane in the world and has claimed many lives of people trying to cross to Britain in inflatable dinghies.
Until today, it was thought at least 10 people had died in the past few weeks while attempting to make the crossing.
Earlier this week, Home Secretary Priti Patel described the number of illegal migrants departing France as “unacceptable”.
She said the government’s Nationality and Borders Bill was the “long-term solution” to tackling the issue “on top of a raft of operational and diplomatic work”.