The United Nations Refugee Agency is formally establishing a full-time presence in the French port of Calais.
It is assisting hundreds of migrants, refugees and asylum seekers who are living in squalid settlements while hoping to cross to the UK.
The UNHCR left Calais in 2002 after the authorities closed the Sangatte reception centre.
It is now providing information to the 1,500 migrants living around the area, including on UK asylum policy.
It has been working there since early June with aid partner France Terre d'Asile to provide those living there with information.
Migrants are told that if they make it to the UK and are caught, they could face a return to the continent or to their country of origin, unless granted asylum.
Gavin Hewitt visits the camps
The UNHCR describes relations between the foreigners and the people of Calais as "tense".
Although the Red Cross centre at Sangatte was closed in November 2002, scores of makeshift insanitary illegal camps have sprung up in its place.
The French and British governments are currently discussing the creation of a new immigrant holding centre within the British side of the Calais docks.
This would allow the countries to send illegal immigrants home more easily.
The UK Borders Agency said the illegal migrants in France were not queuing to get into Britain, they had been "locked out".
A spokesman said: "Last year alone UK Border Agency staff at our French and Belgium controls, not only searched more than one million lorries but also stopped 28,000 attempts to cross the Channel illegally.
"The message is clear that the only way to live and work in France and the UK is by legal routes. Working alongside our French colleagues, we will continue to offer support and humanitarian assistance to those who genuinely need it, but if people are not in need of protection we expect them to return home."
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