London: More than 7,000 illegal migrants have been arrested while trying to sneak into Britain from Calais since January — twice as many as in the same period last year.

And many of them are being released immediately as detention centres are overcrowded.

Despite tougher security measures, new figures from the French authorities yesterday revealed a huge rise in the number trying to make their way through the Channel port to the UK.

Between January and last month, 7,414 were arrested. This compares to 3,129 in the same period last year. Most said they wanted asylum in Britain, which would let them claim benefits.

A source at the Calais port said: “There are considerably more illegal migrants.”

Around 1,200 were arrested in the first two weeks of this month alone, said the source, with most freed straight away. They can then return to Calais and try to board lorries or ferries.

Last week the town’s mayor banned the setting up of makeshift camps, saying they were a danger to public health.

Most of the migrants are from African countries such as Somalia and Eritrea as well as war-torn Middle Eastern nations such as Iraq and Syria.