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.
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.
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.