The number of migrants camped on France's north coast near Calais has doubled to around 6,000 in recent weeks, boosted by an influx across Europe's borders, the government's top official for the region said on Friday.
Many were finding it increasingly difficult to travel on to Britain and were deciding to try and stay in the area for longer periods or even settle there, Pas-de-Calais department prefect Fabienne Buccio told Reuters.
"I do not know what comes next, but 6,000 is the most we can take," she told Reuters, warning other to stay away from the French port city, whose approach roads and railways have been surrounded by fences and razor wire.
"Migrants and people smugglers need to learn that Calais is a dead-end street," she said.
Britain's interior ministry declined to comment on the growing numbers waiting across the water in France.
Thousands fleeing war and poverty in Africa and the Middle East live in a so-called "jungle" of camps in Calais and have tried repeatedly to enter Britain by jumping onto lorries, hiding on trains and walking through the tunnel in the hope of better lives there than in continental Europe.
Sixteen of them have died in the attempt, including one hit by a freight train on Thursday night.
The number of people in the "jungle", having stabilized at over 3,000 during the summer, had risen sharply in the past few weeks, said Buccio.
"When I started this job early this year they said it took on average three months to get to the UK. Now, we see that some have been here for eight months and some are not even trying to go across anymore, some are settling here," she said.
Buccio said authorities were having some success persuading migrants to apply for asylum instead.
Pas-de-Calais was receiving more asylum requests than any other French department - 2,100 since the start of the year, up from 900 last year and 300 in 2013, she said.
She said that for Eritreans and Syrians the acceptance rate was 100 percent, but some Iraqis and Iranians had been sent back.
Buccio said plans were also advancing to improve accommodation as the winter approaches.
France announced plans in August to build shelters for as many as 1,500 migrants. Buccio said this would probably in the form of used shipping containers, since the area's sand dunes could not take traditional foundations.
Buccio said that depending on how popular these dwellings prove, a further 1,500 may also be provided.
"Some of them will not want to come. The new facilities will be regulated and orderly, and some do not like that," she said.
About 170,000 migrants entered the European Union without the right documents or via illegal entry points in September, the bloc's border agency Frontex said on Tuesday, taking the total for the year so far to 710,000.
European leaders, who have struggled to find a joint strategy to cope with the surge of people arriving at their borders, were discussing the crisis at a summit in Brussels on Friday.
(Additional reporting by Kate Holton in London; Writing by Geert De Clercq; Editing by Andrew Callus and Andrew Heavens)