Boris Johnson has warned those thinking of crossing the Channel illegally that they will be sent back to France after dozens of people, including children, were intercepted on Thursday in several incidents at sea and on the south coast.

The prime minister’s comments came as the home secretary, Priti Patel, prepared to hold talks with her French counterpart about the crossings, which have sometimes been linked to fairer weather conditions.

“Clearly the most important thing is to stop them coming across from France so we are working very closely with the French authorities,” the prime minister said during a visit to Devon.

“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. The UK should not be regarded as a place where you could automatically come and break the law by seeking to arrive illegally. If you come illegally, you are an illegal migrant and I’m afraid the law will treat you as such.”

Patel’s predecessor as home secretary, Sajid Javid, declared a “major incident” after an increase in the number of people attempting the journey across the Channel at Christmas. More than 1,000 had attempted the crossing this year, he said in July.

Dozens of people who were picked up at sea and on land by UK authorities on Thursday included 11 people, among them children, who were on two boats which landed at Winchelsea Beach, near Rye in East Sussex.

But there was concern for the welfare of several other people, which Sussex police said could include a family with children and who may have left the beach.

The chief inspector of Sussex police, Anita Turner, said: “We want to make sure that these people are safely ashore and that they are OK.”

A total of 53 people, including at least six children, were picked up in the Channel from a number of small boats while trying to reach the south coast of England on Thursday.

Channel migrant crossings: who is coming and why? Read more

The Home Office provided new details on Friday of one of the incidents on Thursday, which involved a small boat that was escorted to Dungeness by Hastings RNLI. A group of 16 people were taken to Rye police station, where they said they were Vietnamese and Iranian nationals.

The Home Office, which blames criminal gangs for facilitating the crossings, has a number of Border Force cutters in the Channel and a joint action plan with France in place, including a coordination centre in Calais.

French authorities were involved in dealing with two “small boat” incidents on Thursday in which 30 people were recovered and returned to France. They included 10 children and a baby.

On the British side, the Home Office said earlier that the Border Force was alerted at 2.30am on Thursday to a small boat travelling towards the English coast. 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 male passengers, which was said to include two children and people who 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. A later incident, at 11.30am, involved the Border Force intercepting a vessel with a group of 11 people, including four children. All claimed to be Iranian nationals.