Prime Minister David Cameron faced heavy criticism on Thursday for saying a "swarm" of migrants was trying to come to Britain, as authorities in France struggle to stop them from crossing the English Channel.

"You have got a swarm of people coming across the Mediterranean, seeking a better life, wanting to come to Britain because Britain has got jobs, it's got a growing economy, it's an incredible place to live," Cameron told ITV television during a visit to Vietnam.

About 3,000 people from countries that include Syria and Eritrea are camping out in the northern French port of Calais and trying to cross into Britain illegally by clambering on board trucks and trains.