French authorities said Thursday that more than 150 migrants trying to reach Britain had been rescued from overloaded boats in the Strait of Dover and returned to France.

"Two migrant vessels in trouble" in the English Channel strait were reported early Thursday, maritime officials said in a statement.

The first boat had departed from the Gravelines area near Dunkirk before seeking help after a few hours at sea, with rescuers taking on 77 people who were returned to Calais, the Premar maritime authority said.

A second vessel was rescued with 76 people on board who were brought to the French port of Boulogne-sur-Mer, where they were received by border agents.

The attempts to cross the Channel come as political parties in Britain campaign for a general election on July 4 in which immigration is one of the major issues.

Earlier this month, more than 880 irregular migrants crossed the Channel to Britain on small boats, the highest single-day total so far this year, the UK government said.

An estimated 12,313 people have made the crossing to Britain so far this year, an 18 percent increase from the same period last year, the UK Home Office said.

At least 15 migrants have been killed attempting to cross the Channel so far this year, topping the 12 killed in the whole of last year, according to official figures.