“The rhib (rigid-hulled inflatable boat), with 20 people on board was located at 2.00am (0100 GMT) and the incident handed over to Border Force,” it said in a statement.
Thousands of people have been massed in northern France for months as they try to reach Britain where the believe they will have a better chance of finding employment, according to French and British charities.
Calais coastguard organisation SNSM assisted in the rescue operation, according to its president Bernard Barron.
“We were called for help… to search between Calais and Dover for a boat carrying about twenty people,” he told AFP.
Helicopters from England and France took part in the search.
The passengers were taken to the nearby port of Dover to be interviewed by Border Force officers, the Home Office said.
“The castaways, who were migrants, called their families, who then alerted the authorities and rescue missions were triggered on both sides of the Channel,” said Barron.
“This confirms our fears: the smugglers are willing to take extreme measures, but the Channel is a real highway, presenting a great danger for this type of crossing,” he said.
Some 17 suspected Albanian migrants were detained Tuesday after a boat arrived at Chichester Marina on the south coast.
Four Iranian migrants making a rare attempt to reach Britain by boat from France in February were rescued just in time after their vessel took on water and was close to capsizing.
Rescuers were only alerted after one of the migrants made it back to the beach at Sangatte in northern France before dawn.
Officials warned last month that migrants from the Jungle camp in Calais were stepping up attempts to reach Britain with summer on its way, with a sharp increase in migrant attempts to stow away in the back of lorries.
The government says the figure is vastly inflated, but tension remains high as police regularly jostle with migrants desperate to cross the Channel.