Boris Johnson has suggested changing the law to make it easier to send back migrants who have made the perilous Channel crossing to the UK.
The Prime Minister said it was “bad and stupid and dangerous and criminal” for desperate people to travel to the UK in small boats.
Campaigners accused Mr Johnson of using “inflammatory” soundbites as tensions escalated over the influx of asylum seekers arriving in the UK.
More than 4,000 people are believed to have come to Britain so far this year, making the treacherous journey across the world’s busiest shipping lane.
The Government has been accused of being “increasingly chaotic” in its handling of the situation – with anger over Home Secretary Priti Patel’s request to bring in the Navy to deal with the number of small boats.
But Mr Johnson ramped up his rhetoric on the issue, saying the Government would look at the law to make it easier to send migrants home again.
Speaking on a visit to a school in Essex on Monday, he said: “Be in no doubt what’s going on is the activity of cruel and criminal gangs who are risking the lives of these people taking them across the Channel, a pretty dangerous stretch of water in potentially unseaworthy vessels.
“We want to stop that, working with the French, make sure that they understand that this isn’t a good idea, this is a very bad and stupid and dangerous and criminal thing to do.
“But then there’s a second thing we’ve got to do and that is to look at the legal framework that we have that means that when people do get here, it is very, very difficult to then send them away again even though blatantly they’ve come here illegally.”
No10 said Brexit would make it easier to create new rules for dealing with migrants, claiming the UK was bound by “inflexible and rigid” EU regulations, which decide which country is responsible for considering asylum claims.
Refugee campaigners condemned the Government’s rhetoric on asylum seekers and called on the Prime Minister to avoid “scapegoating” desperate people.
Lisa Doyle, director of advocacy for the Refugee Council, said: “It’s incredibly disappointing to hear the Prime Minister using such inaccurate and inflammatory language to describe men, women and children who are desperate enough to make perilous journeys across the busiest shipping channel in the world.
“Seeking asylum is not a crime, and it is legitimate that people have to cross borders to do so.”
She urged ministers to avoid “scapegoating people in desperate circumstances” and instead to provide better legal routes to claiming asylum.
Stephen Hale, chief executive of Refugee Action, told the Government to “move on from soundbites”.
He said: “Britain is better than this.
“We have a proud history of welcoming people fleeing some of the most violent and oppressive regimes in the world and we can’t stop now.”
Immigration Minister Chris Philp is due to hold talks with his French counterpart on Tuesday amid tensions with Paris over policing of the Channel.