French riot police evacuated migrants from a warehouse in Calais today after ministers expressed concern that the building could become a base for those trying to reach Britain.

Up to 90 migrants, mainly Afghan, had broken through a police cordon to gain access to the warehouse, which was rented by left-wing activists opposed to the French Government's immigration policies.

The incident created a political storm in France and placed the issue of migration to Britain back in the headlines

However, many of the migrants left the warehouse in search of food on their own and the rest were evicted by about 75 police officers.

''The police smashed up everything, there are no more migrants in there,'' said an activist.

The incident occurred less than five months after officials ordered the destruction of The Jungle, a makeshift camp in Calais that was home to hundreds of migrants hoping to cross the Channel to Britain.

In a statement, Eric Besson, the Immigration Minister, said that he would not let activists create a ''new Jungle ... serving as a rear base for human trafficking gangs near the port of Calais.

Natacha Bouchart, the Mayor of Calais, said that the migrants had only got into the building because of a ''flaw'' in policing.

Officials had known that No Border, a hard-left migrant support group, was planning to use the building as a shelter and had ordered police to seal off the road that led to it.

But officers let the migrants through on Saturday after activists said that they were holding a party and would clear the warehouse when it had finished.

Instead, the migrants spent the night there.

The stand-off came with French officials hailing the destruction of the Jungle as a decisive blow in the struggle against the people-trafficking networks that transport migrants from Calais to Dover.

They say that the number of migrants in the French port has fallen from more than 1,000 to fewer than 300 since the operation in September.

''There has been a considerable improvement and the numbers we now have are much more manageable,'' said Mrs Bouchart.