The government's increasingly shrill tone has upset charities, churchmen and left-wing politicians, while anti-EU campaigners have seized on the crisis as proof that Britain is unable to protect its own borders while remaining in the bloc.

Cameron, criticised by political rivals for referring to the migrants as "a swarm", won an unexpectedly decisive election victory as recently as May.

But the Calais events - in which hundreds of migrants from North Africa and the Middle East try nightly to scale fences - pose a political problem by spotlighting his failure to keep his promises on controlling immigration. That may make it harder for him to persuade voters to stay in the EU.

Cameron has promised to overhaul Britain's EU ties before holding a referendum on membership by the end of 2017. He has also vowed to cut net annual migration to the tens of thousands from the hundreds of thousands but repeatedly missed the target.

Calais has long been a magnet for migrants seeking to try their luck at slipping into Britain, a country without identity cards where it is easier than on the continent for workers to melt into a booming informal economy.

What is different this time is that the migrants number thousands not hundreds, and disruption to freight and passenger traffic has been prolonged in the summer holiday season.

French police union official Gilles Debove told Reuters some 1,700 attempts to enter the tunnel had been recorded overnight and a French riot policeman had been injured by stone-throwing.

The crisis has proved fatal for some migrants, nine of whom have been killed trying to make the crossing. Now some French politicians are talking of scrapping treaties that allow Britain to operate border controls on the French side of the tunnel.

Indeed one leading conservative Gaullist, Xavier Bertrand, suggested at the weekend that if Britain did not agree to start processing the asylum seekers, France should simply let them loose to try to breach the border.

The British government has responded by holding a series of meetings of its emergency response committee, known as Cobra, and by pledging more security fencing and sniffer dogs.

It has also promised to bring forward a law to tighten immigration rules, targeting landlords who rent properties to illegal immigrants with jail sentences, and has floated the idea of stripping families of failed asylum seekers of benefits.

In an article on Sunday, Theresa May, the interior minister, spoke of the need to convey the message that Britain's streets were not "paved with gold".

But the government's approach has angered some.

Church of England Bishop Trevor Willmott warned the government not to forget its humanity.

"When we become harsh with each other and forget our humanity then we end up in these stand-off positions," he told the Observer newspaper.

Habib Rahman, chief executive at the Joint Council for the Welfare of Immigrants, called ideas to remove state support from families of failed asylum seekers the "worst kind of political posturing," saying it would have a wider discriminatory effect.

The Conservatives must tread a fine line. When in opposition in 2002, May warned that some people had begun to call them the "Nasty Party" due to their attitudes towards ethnic minorities. Cameron will not want his party to regain that epithet.

But he is also under pressure from Eurosceptic lawmakers in his own party and the predominantly right-leaning press who want him to take a tougher line on immigration.

"Why are we seeing illegal migrants 'swarming', as he (Cameron) rightly puts it, through razor wire to the Eurotunnel?" The Sun tabloid, part of Rupert Murdoch's media empire, lamented on Monday.

"Nothing more vividly exposes Britain's abject surrender to EU control. Mr Cameron says he will take back our borders and kick out illegal immigrants. It is all talk."

Source : Reuters