The coastguard says it is dealing with "multiple" small boats with migrants on board off the Kent coast.
Dozens of people wrapped in blankets and wearing lifejackets have been seen being brought into the port of Dover as migrants take advantage of calm seas.
On Friday 319 people were picked up as they crossed the English Channel.
Clandestine Channel Threat Commander Dan O'Mahoney said the UK was working with French authorities to "make the route completely unviable".
He added: "This morning I was out on a cutter in the Channel working with Border Force officers dealing with these boats.
"The unseaworthy vessels I saw provided further evidence that these crossings, so often facilitated by criminals, are risking people's lives.
"This morning the French prevented more people from embarking on this perilous journey, but we know the numbers arriving are still too high.
"These migrants are leaving France which is a safe country. Those seeking refuge can and should claim asylum there, rather than attempting these dangerous crossings to the UK."
On Wednesday a committee of MPs was told the majority of unaccompanied children arriving in Kent were 16 and 17-year-old boys.
The Home Office does not provide information on how many children are making the crossing on small boats.