Video report by ITV News Correspondent Paul Davies
Boris Johnson has warned migrants who sail across the English Channel that the UK will "send you back" if they cross illegally.
The prime minister's remarks came after nearly 100 people took to a number of small boats in a bid to cross from France to the UK in the space of 24 hours.
He said: "The UK should not be regarded as a place where you could automatically come and break the law by seeking to arrive illegally."
Campaigners and human rights lawyers said Mr Johnson's comments on migrants are "misleading and inflammatory", adding people have a right to seek assistance in the UK and it was important asylum applications were not "pre-judged".
Speaking on Friday, Mr Johnson said: "Clearly the most important thing is to stop them coming across from France so we are working very closely with the French authorities.
“The point I would just make to people thinking of making this journey – one, it is very hazardous, you may think the weather looks great but it’s a very, very dangerous thing to do.
“The second thing is – we will send you back.
“If you come illegally, you are an illegal migrant and I’m afraid the law will treat you as such.”
Home Secretary Priti Patel held talks with her French counterpart on Friday in a bid to resolve the developing crisis.
The Joint Council for the Welfare of Immigrants said the prime minister's comments were "misleading and designed to inflame tension" and "if put in practice, would violate international law."
"The UK is perfectly capable of providing safety and a well-functioning, dignified welcome for desperate people trapped in limbo across Europe," a spokesperson said.
"Those who are forced to take dangerous means of reaching our shores have a legal right to have their voice heard and we should be creating safe and legal routes for vulnerable people to come."
Annette Elder, a partner at human rights law firm Elder Rahimi and a member of the Law Society's immigration committee said Mr Johnson's comments demonstrated how "asylum seekers are criminalised."
She said: "Clearly it's not possible to say that everyone who crosses the Channel in small boats from France is going to be sent back.
"The UK is bound by legal obligations.
"This is another example of the way asylum seekers are criminalised. To talk about people illegally crossing the Channel - it's inflammatory.
"It misrepresents the situation. Everyone has a right to seek asylum."
Timeline of recent migrant findings:
At 2.30am on Thursday, Border Force saw small boat travelling towards UK coast. Group of eight men and one woman, all claiming to be Iranian nationals, were found and taken to Dover
At 5am, Border Force intercepted another small boat of 11 males, which was taken to Dover. The group included two who said they were children. They said they were nationals of Iran, Guinea, Kuwait, and the Ivory Coast.
At 7am, six men, who claimed to be Iranian nationals, were spotted on a small boat and taken to Dover.
At 11.30am, Border Force sent to intercept boat carrying 11 people, including four who said they were children. All claimed to be Iranian nationals.
At the same time, Border Force called to Winchelsea beach in East Sussex, where 11 migrants were found.
At around 12pm, officials called to another small boat crossing the Channel. The boat was escorted to Dungeness. Some 16 people, claiming to be of Iranian and Vietnamese descent, were handed over to immigration officials.
At least seven crossings were intercepted on Thursday involving 94 people trying to cross the Channel from France to England.
The crossing have continued into Friday, with UK authorities called to the Kent coast near Lydd.
The French coastguard said 24 migrants, reportedly including seven children and a woman, were found crossing the Channel.
Dover MP Charlie Elphicke said on Twitter: “Only when migrants and traffickers alike know that they will not succeed in crossing the English Channel will this crisis come to an end.”
The Home Office now confirmed officials dealt with 64 people, including at least six children, who tried to reach England in small boats in six separate incidents on Thursday.
French authorities said it found 30 migrants, including 11 children, attempting to make the crossing that same day.
They were all taken back to the continent, according to the Home Office.
The crossings came as official figures were published on immigration to the UK from the EU. Data showed the number of people offered asylum and humanitarian protection by the UK has risen to its highest level since 2003.
A Home Office spokeswoman said attempting such crossings was a “huge risk” and “the criminal gangs who perpetuate this are ruthless and do not care about loss of life”.
She added: “We continue to monitor the ongoing situation and are working closely at all levels with French authorities on the issue.
“The Home Secretary will raise this issue with her French counterpart in the coming days.”