A group of 15 Iranian men were found on board two small boats as they tried to cross the English Channel.
Four of them have been arrested on suspicion of facilitating illegal entry to the UK, the Home Office said.
Border Force dealt with two “incidents involving small boats” off the Kent coast, a spokeswoman said.
Officials were first called at 9am on Monday.
A cutter and a patrol boat were sent to the scene and nine of the men were brought to Dover.
Six were handed to immigration officials and the remaining three were arrested on suspicion of facilitating illegal entry to the UK.
At midday officials were called again after the sighting of a second small boat.
The six men found on board were also taken to Dover, five were handed over to immigration officials and one was arrested.
The group, wearing life jackets and covered in blankets, were seen being brought ashore at Dover Marina from a Border Force cutter on Monday afternoon.
All of them claimed to be from Iran and were “medically well”, a spokeswoman said.
On Sunday, a family of seven was found near Kingsdown after an empty dinghy was spotted in the Channel.
The mother, father and five children were handed over to immigration officials.
At the time a Home Office spokesman said: “Since the Home Secretary declared a major incident in December we have tripled the number of cutters operating in the Channel, agreed a joint action plan with France and increased activity out of the Joint Coordination and Information Centre in Calais.”
The number of people attempting to cross the Channel dropped from roughly 250 in December to 90 in January, with about half of the January attempts being intercepted in France before reaching British waters, the spokesman added.
A week ago a group of 34 men, women and children were detained after cutters and a coastal patrol boat intercepted a dinghy.