The mayor of Calais has blamed Britain for leaving it alone in facing problems created by the thousands of migrants who arrive in the port city hoping to cross the Channel.

Natacha Bouchart accused the UK of an artful commitment the EU, claiming London was cherry picking the some aspects of the Union, without getting fully involved in it.

"A country cannot remain both in and out of Europe, creating problems but providing no compensation and no support to local authorities such as mine and others, which are left alone to assume huge responsibilities," she told the Council of Europe's Journal, according to The Guardian.

Bouchart, first elected in 2008 with the centre-right UMP party of Nicholas Sarkozy, said the source of the Calais immigration problem was Britain's decision not to sign the Schengen agreement on open borders.

"The UK is not really interested since border controls take place on French soil," she said. "It provides no support to the city of Calais, nor to the local population, and it leaves the mayor of Calais to manage the problem without financial assistance. It's truly an enormous responsibility and hugely difficult for me personally as mayor of a town of 75,000 inhabitants," Bouchart added.

Since the signing of the so-called Le Touquet Treaty in 2003, British border controls have been moved from Dover to Calais and vice versa. Bouchart repeated claims made last year before lawmakers in London that migrants are attracted to Britain by its generous benefit system.

"They want to go to England because they can expect better conditions on arrival there than anywhere else in Europe or even internationally. There are no ID cards. They can easily find work outside the formal economy, which is not really controlled. They can get social welfare support that doesn't exist in other countries," said Bouchart.

The Calais migrant population has swelled from a few hundred to about 3,000 in the last 12 months due to wars and security crises in North Africa and the Middle East. Local charities have reported that the number has almost halved over the last few weeks, due to a series of factors including a high success rate in illegal crossings to the UK.

"The numbers of migrants diminished to about 1,200 in the last few weeks, due to much success making it to England, more asylum seekers getting lodging outside of Calais and fewer arrivals," charity L'Auberge des Migrants said earlier in March.

Numbers are expected to rise again in the coming months, with the milder spring weather easing travel conditions.