PARIS (REUTERS) - British and French officials traded blame after 27 people died on Wednesday (Nov 24) when their dinghy capsized while crossing the Channel from France to Britain.

The accident was the worst disaster on record involving migrants in the narrow seaway separating the countries.

The Channel is one of the world's busiest shipping lanes and currents are strong. Overloaded dinghies often barely stay afloat and are at the mercy of waves as they try to reach British shores.

More migrants left France's northern shores than usual to take advantage of calm sea conditions on Wednesday, according to fishermen, although the water was bitterly cold.

One fisherman called the rescue services after seeing an empty dinghy and people floating motionless nearby.

British Prime Minister Boris Johnson said he was "shocked and appalled" by the deaths and called on France to do more to deter people from attempting the crossing. People trafficking gangs were "literally getting away with murder", he said.

President Emmanuel Macron said Britain needed to stop politicising the issue for domestic gain, while his interior minister, Mr Gerald Darmanin, said Britain too had to be a part of the answer.

France had earlier stated 31 people lost their lives, but the number was later revised down to 27, government officials said.

Mr Darmanin said he was heading for the coast. "Strong emotion in the face of the tragedy of numerous deaths due to the capsizing of a migrant boat in the English Channel," he wrote in a tweet.

Two migrants were critically ill in hospital with severe hypothermia, he said.

French police arrested four human traffickers suspected of involvement in the accident. Mr Darmanin said the nationalities and identities of the migrants were not known.

Three helicopters and police and rescue boats were still at the scene, looking for people missing from the capsized vessel, said Maritime Minister Annick Girardin.

The local coast guard said they could not yet confirm the total number of deaths.

One fisherman, Nicolas Margolle, told Reuters he had seen two small dinghies earlier on Wednesday, one with people on board and another empty.

He said another fisherman had called rescue services after seeing an empty dinghy and 15 people floating motionless nearby, either unconscious or dead.

He confirmed there were more dinghies on Wednesday because the weather was good. "But it's cold," Margolle added.

Mr Darmanin said the migrants' dinghy had deflated, and when rescuers had arrived it was "deflated like an inflatable garden pool".

Early on Wednesday, Reuters reporters saw a group of over 40 migrants head towards Britain on a dinghy.

While French police have prevented more crossings than in previous years, they have only partially stemmed the flow of migrants wanting to reach Britain - one of many sources of tensions between Paris and London.

In his statement, Mr Johnson said he and Mr Macron had agreed to step up efforts to prevent the crossings.

Some rights groups said that tighter monitoring was pushing migrants to take greater risks as they sought a better life in the West.

"To accuse only the smugglers is to hide the responsibility of the French and British authorities," l'Auberge des Migrants NGO said.

Before Wednesday's disaster, 14 people had drowned this year trying to make it to Britain, a local maritime prefecture official said. In 2020, a total of seven people died and two disappeared, while in 2019 four died.