The Royal Navy is taking over operational command from Border Force in the English Channel from today.
The service will be responsible for intercepting boats carrying migrants.
The Prime Minister said the new offshore asylum approach is intended to end the "barbaric trade in human misery conducted by the people smugglers in the Channel" and said crossings could reach 1,000 a day in a few weeks.
"These vile people smugglers are abusing the vulnerable and turning the Channel into a watery graveyard," Boris Johnson said.
"To identify, intercept and investigate these boats, from today the Royal Navy will take over operational command from Border Force in the Channel, taking primacy for our operational response at sea in line with many of our international partners with the aim that no boat makes it to the UK undetected."
Mr Johnson added that giving the Royal Navy operational command in the English Channel will send a clear message to criminal gangs.
"This will be supported by £50m of new funding for new boats, aerial surveillance and military personnel," the Prime Minister added.
Watch: In January, the Defence Committee heard evidence about the military's involvement in monitoring migrant crossings.
"In addition to the existing task force of patrol vessels, helicopters, search and rescue aircraft, drones and remotely piloted aircraft, this will send a clear message to those piloting the boats: if you risk other people's lives in the Channel, you risk spending your own life in prison.
"People who do make it to the UK will be taken not to hotels at vast public expense, rather they will be housed in accommodation centres like those in Greece with the first of these to open shortly."
The decision to task the military with policing the Channel has received some criticism in the past.
In January, the Defence Select Committee was told there was "no spare capacity" to deploy naval vessels to police the English Channel, while the same month the committee's chair, Tobias Ellwood, told Forces News his belief that: "That's not what our Navy is for."
A Defence Select Committee report in March also called for a "clear end point" to the British military's role in tackling the number of migrant crossings in the Channel.