Boris Johnson said he was “shocked” and “appalled” that 31 migrants drowned in the Channel after their inflatable boat sank in the worst disaster on record involving migrants in that stretch of water.
The prime minister urged France to step up efforts to stem the flow of people crossing to the UK in small vessels.
French police have arrested four suspected traffickers on suspicion of being linked to people-smuggling.
The country’s interior minister Gerald Darmanin said 34 people had been on board, 31 of whom had died, two were rescued and one was still missing.
“There are two survivors ... but their life is in danger - they are suffering from severe hypothermia,” he said.
More migrants left France than usual to take advantage of calm sea conditions on Wednesday, according to fishermen, although the water was bitterly cold. A joint French-British search operation for survivors is still under way.
Although both governments blamed smugglers, a number of French politicians blamed Britain for the problem, saying it should change its immigration policies.
More than 25,700 people have made the dangerous sea journey from France to the UK so far this year, more than three times higher than the total last year.
