Sixty-four migrants including a pregnant woman and several children were rescued in the English Channel in the past 24 hours as they were seeking to reach Britain aboard rickety vessels, French maritime authorities said Sunday.

A first group of 45 migrants were spotted Saturday off of Leffrinckoucke, near the northern French port of Dunkirk, aboard a small craft "in difficulty", a first statement by the French prefecture in charge of the Channel and North Sea said.

Several "appeared to be in a state of hypothermia" after being brought aboard a patrol ship, it said.

First responders and border police took charge of the migrants, who were "all safe and sound" when they were put ashore in Calais in the early evening, it added.

Then early Sunday a French customs patrol boat rescued a second group of 19 migrants some 18 kilometres (11 miles) south of Boulogne-sur-Mer. They included at least two children, who also "seemed to be in a state of hypothermia," Sunday's statement said.

Migrants attempting to cross the Channel in a small boat in August Migrants attempting to cross the Channel in a small boat in August Photo: AFP / Sameer Al-DOUMY

They were handed over to police, it added.

Calm weather this weekend prompted fears among French sea rescuers that larger numbers than usual would attempt the crossing to Britain, a source close to emergency services told AFP.

With busy traffic and powerful winds and currents, the Channel crossing can prove extremely dangerous for small boats despite the apparent short distance between Calais and British port Dover.

In late October, four people died and three were reported missing, devastating an Iranian-Kurdish family in the deadliest single incident for migrants attempting to cross the Channel.

Increasing numbers of people have attempted the sea crossing from France to Britain since 2018.

On Saturday, the two countries signed an accord aimed at shutting down the route, which calls for French patrols to be doubled and for radar and drones to be used to spot migrant boats earlier.