- Europe migrant crisis
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The UK and France will keep "all options on the table" as they try to stop migrants crossing the Channel.
The UK prime minister and French president said trafficking gangs were risking lives, after 27 people died on Wednesday in the worst-recorded migrant tragedy in the Channel.
Four people have been arrested in connection with the fatal crossing.
Five women and a girl were among the dead, France's interior minister, Gerald Darmanin, said.
Two people were rescued and one was missing, he also said.
It was earlier reported 31 people had died, but the total was revised down overnight into Thursday.
Boris Johnson said he was "appalled" by what happened in the Channel on Wednesday, adding the UK would leave "no stone unturned" to stop human trafficking gangs.
Mr Johnson and Emmanuel Macron also said it was important to work closely with Belgium and the Netherlands, as well as partners across Europe, to better tackle the problem before people reach the French coast, a spokesman for the PM said.
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The alarm was raised on Wednesday after a fishing boat crew spotted several people at sea off the coast of France.
The International Organization for Migration said it was the biggest single loss of life in the Channel since it began collecting data in 2014.
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Speaking after an emergency government Cobra meeting on Wednesday, Mr Johnson said it was vital to step up efforts to break the business model of "gangsters" who send people to sea.
He admitted efforts so far to stem the flow of migrants crossing the Channel in small boats have not been enough and the UK would offer to increase its support to France.
The UK has pledged to pay France €62.7m (£54m) during 2021-22 to help increase police patrols along its coastline, boost aerial surveillance and increase security infrastructure at ports.
By Nicholas Watt, political editor, BBC Newsnight
Speaking to one senior UK government official, it is clear the Channel crossings have been a huge issue in Downing Street.
They say that, over the last 18 months, they have attended more meetings on this in No 10 than any other issue with the exception of Covid.
I understand that the strong thinking in No 10 is for joint UK and French patrols along the French northern coast.
That would mean British security forces and French security forces – and the reason for that is that it is so very difficult to police the entire coastline.
The reason why that is so significant is that there is a feeling that if you can intercept as many as 75%, or even 90% of the boats crossing, it would have a huge impact.
It could destroy the economic model of the smugglers and then break up their network.
Mr Macron said France would not let the Channel become a "cemetery". Since the start of 2021, he said, 1,552 smugglers had been arrested in northern France and 44 smuggler networks dismantled.
Despite this, 47,000 attempted Channel crossings to the UK took place this year and 7,800 migrants rescued, Mr Macron added.
French government ministers and officials will meet later on Thursday to discuss their response to the tragedy.
A number of people are believed to have reached the UK in small boats on Wednesday, with people seen being brought ashore in Dover by immigration officials.
It comes amid record numbers of migrants making the crossing from France to the UK.
The Dover Strait is the busiest shipping lane in the world and has claimed many lives of people trying to cross in inflatable dinghies.
It is thought at least 10 other people had died in the past few weeks while attempting to make it.
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