British prime minister David Cameron has come under fire for describing migrants who are trying to get into the UK from France as a "swarm".

Speaking from the other side of the world, on his South-East Asian tour, Mr Cameron insisted the UK would not become a "safe haven" for illegal migrants from the French port of Calais.

" I accept that you've got a swarm of people coming across the Mediterranean, seeking a better life," he said.

"But we need to protect our borders by working hand in glove with our neighbours, the French, and that's exactly what we're doing."

But Mr Cameron's remark drew strong criticism from opposition figures and human rights advocates.

Acting Labour Party leader Harriet Harman said Mr Cameron should remember he was talking about people, not insects.

A contender for Labour's leadership, Andy Burnham, described the word as a dog whistle — a silent call to those opposing migration.

Even Nigel Farage, the UKIP Party leader whose support is built on such sentiment, distanced himself from the term when talking to the BBC.

"I'm not seeking to use language like that," he said.

Migrants walk along railway tracks at the Eurotunnel terminal in Calais. ( AFP: Philippe Huguen )

But just a short time later, he used the word himself on ITV.

"A couple of times I've been stuck on the motorway and surrounded by swarms of potential migrants to Britain," he said.

Dr Lisa Doyle from the Refugee Council said using the word swarm was dehumanising and shifted the blame away from politicians who have mishandled the crisis.

"It's dehumanising, and its ramping up negative rhetoric," Ms Doyle said.

"'What it's doing is shifting the blame from Europe who are not making policies to support people."

The UN Special Representative for International Migration has accused British politicians of adopting a "xenophobic response" to the issue of free movement, labelling the reaction "grossly excessive".

Hundreds of migrants have attempted to make a dash past French police towards trucks and trains to stow away to England.

One person was found dead on Wednesday following an attempt by 1,500 migrants to enter the Eurotunnel terminal in Calais. It was the ninth death of a migrant near the terminal site since June.

Pressure is growing on the British and French governments to find a solution, with no end in sight to long delays caused by the migrants' activities, and a plea from the council near the British coast for help with growing numbers of unaccompanied minors.

But Ms Doyle said Mr Cameron's words were disproportionate.

"He talked about the people coming across the Mediterranean with the express intent of coming to Britain and that's not the case," she said.

"Two-hundred-thousand people have crossed the Mediterranean this year and we haven't seen an increase in asylum applications anywhere near that proportion.

"There were an estimated 3,000 people at Calais. You compare that to 200,000 people. We are not playing our part. Britain has fewer than 1 per cent of the world's refugees."

ABC/wires