Téteghem Mayor Franck Dhersin, the vice president of transport for the northern France region, told Reuters that the death toll had risen to 31 on Wednesday evening after a boat carrying migrants across the channel capsized.
Dhersin had tweeted earlier that more than 50 migrants were believed to have been on board.
"Lots of corpses in the water," he wrote, calling the incident "a new tragedy."
British Prime Minister Boris Johnson said he was “shocked, appalled and deeply saddened by the loss of life at sea."
"My thoughts are with the victims and their families," he said as he called for countries to work together to stop smugglers from facilitating channel crossings.
"Now is the time for us all to step up, work together and do everything we can to stop these gangs who are getting away with murder," he said.
A fisherman, Nicolas Margolle, told Reuters that he had seen at least two small dinghies earlier Wednesday.
He said that one of the boats appeared to have people on board and that the other appeared to be empty.
Margolle said another fisherman called rescue operators after having spotted the empty dinghy and seeing several people floating in the water nearby.
He said there appeared to be more dinghies in the water Wednesday because the weather was good. "But it's cold," he said.
Tensions have risen over small boat crossings on the channel, with French and British officials ramping up efforts to block asylum-seekers from making the journey.
Despite efforts on both sides, the daily number of people crossing the channel on small boats reached record numbers this month, with 1,185 people having arrived in the U.K. on Nov. 11 alone, Sky News reported.
The number was reported to have surpassed the previous record of 853 people, which was reached earlier in the month. A spokesperson for the Home Office said it could not comment on the reported numbers of crossings.
Rise in deaths at sea
The United Nations' migration agency warned in a report in July that the number of migrants, asylum-seekers and refugees who have trying to reach Europe by sea more than doubled in the first half of the year.
In the first six months of the year, at least 1,146 people died while risking the journey to European shores, according to the report by the International Organization for Migration.
In recent months, Britain's Home Office has ramped up efforts to crack down on irregular migration. Home Secretary Priti Patel introduced a bill in July to deter migrants from crossing the channel to reach the U.K.
The British government has said the measure, dubbed the Nationality and Borders Bill, is intended to "better protect and support those in genuine need of asylum," to "deter illegal entry into the U.K." and to make it easier to remove "those with no right to be here."
Immigration advocates and politicians have condemned the bill, and many have called on the U.K. to focus on creating safe routes for asylum-seekers and migrants to reach Britain instead.
“Human beings will continue to drown in the Channel while the Government pursues a dangerous and reckless refugee policy that prioritizes tough borders and punishment instead of protection," Tim Naor Hilton, the chief executive of the immigration advocacy group Refugee Action, said in a statement.
In a statement published Wednesday on Twitter, British MP Zarah Sultana said she was "heartbroken" by the news of the deaths on the channel.
"Please let this be the moment we provide safe routes to welcome refugees to Britain, instead of endlessly whipping up hate and fear," wrote Sultana, a member of the opposition Labour Party.
In a statement, Patel said her thoughts were "with the families of all those who have tragically lost their lives in French waters today."
She said Britain will "continue to intensify" its efforts to “prevent migrants embarking on these deadly journeys.”
The deadly incident, she said, "serves as the starkest possible reminder of the dangers of these Channel crossings organized by ruthless criminal gangs."
"It is why this Government’s New Plan for Immigration will overhaul our broken asylum system and address many of the long-standing pull factors encouraging migrants to make the perilous journey from France to the United Kingdom," Patel said.
France's coast guard did not immediately respond to a request for comment.