Boris Johnson has warned migrants preparing to cross the Channel and enter Britain illegally “we will send you back”.
The prime minister said anyone who attempted to cross the Channel by unlawful means would be immediately deported, adding “if you come illegally you are an illegal migrant”.
Fears are mounting that the forecast warm weather and calm seas could see hundreds of migrants risk their lives in an attempt to reach UK shores.
The Lib Dems said it was “shameful” and “just plain wrong” for Johnson to label all Channel migrants illegal immigrants.
In four separate events on the south coast since Thursday, the Border Force dealt with a total of 37 people, including six children, who tried to reach England in small boats.
And a fifth incident off the Sussex coast reportedly involved more than 20 others, although the Home Office has not confirmed details.
Home Secretary Priti Patel is preparing to hold talks with her French counterpart.
During a visit to Devon on Friday, the prime minister issued the stark warning.
He 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 think the weather looks great but it is very dangerous thing to do, and the second thing is we will send you back.
“The UK should not be regarded as a place where you can automatically come and break the law by seeking to arrive illegally. If you come illegally, you are an illegal migrant and the law will treat you as such.”
The Lib Dems’ home affairs spokeswoman Christine Jardine said the government should be investing more cash in Border Force operations.
She said: “It is just plain wrong for Boris Johnson to label refugees crossing the Channel as ‘illegal migrants’. Quite apart from the dehumanising language, there is nothing illegal about seeking sanctuary in the UK, and it is shameful that we have a Prime Minister who says it is.
“We know that many of these people have fled persecution in Iran. For the prime minister to casually dismiss their rights to asylum with no evidence whatsoever is unlawful and inhumane.”
At 2.30am on Thursday, the Border Force was alerted to a small boat travelling across the Channel towards the UK coast.
A group of eight men and one woman, who all said they were Iranian nationals, were found and taken to Dover.
At 5am, a Border Force cutter intercepted another small boat and a group of 11 males were taken to Dover.
The group, including two who said they were children, presented themselves as nationals of Iran, Guinea, Kuwait, and the Ivory Coast.
Two hours later six men, who said they were Iranian nationals, were spotted on a small boat and taken to Dover.
And at 11.30am, the Border Force was deployed to a vessel with a group of 11 people, including four who said they were children, who all claimed to be Iranian nationals.
In Sussex, police believe that a number of people may have left a beach following the landing of two boats containing suspected illegal immigrants at Winchelsea, near Rye, on Thursday.
A group of adults and children were transferred to immigration officials after the boat, believed to have crossed the Channel, landed between Dogs Hill and Rye Harbour.
However, it is thought that some them, possibly a family including children, may have left the beach on their arrival and there is concern for their safety and welfare.
The Office for National Statistics published new data on Thursday which showed that immigration to the UK from the EU was at its lowest level since 2013. Net migration to the UK was 226,000 in the year to March, the ONS said.