French President Emmanuel Macron and UK Prime Minister Boris Johnson both expressed horror at the tragedy, and sent their condolences to the families of the victims.

Macron said his country would not let the English Channel become a graveyard and urged his European counterparts to increase efforts to avoid future tragedies.

"To the families of the victims, to their loved ones, I want to express my compassion and the unconditional support of France," the President said in a statement.

"I assure them that everything will be done to find and condemn those responsible, networks of smugglers who exploit misery and distress, endanger human lives and ultimately decimate families," he added.

Macron said France had been working with the UK for several months to dismantle smuggling networks.

"Since the beginning of 2021, thanks to the mobilization of 600 police officers and gendarmes, 1,552 smugglers have been arrested on the northern coast and 44 smuggling networks have been dismantled," he said.

"Despite this action, 47,000 attempts to cross to Great Britain have been made since January 1st, with 7,800 migrants saved by our rescue services."

France's regional maritime prefecture earlier said a search-and-rescue operation for survivors was ongoing in the Strait of Pas-de-Calais, and described it as the worst-ever accident in the stretch of water.

The country's sea minister Annick Girardin said French, British and Belgian helicopters were searching for the missing.

French Prime Minister Jean Castex described the incident as a "tragedy." "My thoughts are with the many missing and injured, victims of criminal smugglers who exploit their distress and misery," he said in a tweet.

Dangerous crossing

UK Prime Minister Boris Johnson said he was "shocked and appalled, and deeply saddened by the loss of life at sea."

"My thoughts and sympathies are, first of all with the victims and their families, and it's an appalling thing that they have suffered," Johnson told reporters.

"But I also want to say that this disaster underscores how dangerous it is to cross the Channel in this way.