The migrant crisis in Calais is now so bad that authorities are considering extending the Channel Tunnel by a third of a mile to stop migrants stowing away on trains to Britain.

A section of track from the opening of the tunnel to near the freight terminal will be encased in concrete, in a project costing around £4.7 million.

The decision comes amid fears of a new wave of migrants entering Europe over the summer, with thousands expected to travel to northern France and gather outside the Channel Tunnel terminal in the hope of smuggling themselves into Britain.

Jacques Gounon, chief executive of Eurotunnel, told The Times: “The risk this summer is that we will see migrants overwhelming Europe.

“I would like to not be reactive, like last July, but to anticipate the problem and have additional security measures.”

He also warned there is a danger that ministers “say there is no migrant issue so there is no reason to fund some additional protection”.

The extension could be built over a four-month period, requiring new signalling, emergency exits and firefighting equipment to be constructed.

Another measure also under consideration is a joint control and command centre staffed by officials from both France and Britain which is due to be set up in the coming months.

As well as the £4.7 million construction cost for the tunnel extension, the taxpayer is also facing a bill for £22 million in compensation to Eurotunnel for disruption caused by migrants storming the tunnel.

The chaos was particularly bad last summer when trains from had to be turned back and riot police deployed as hundreds of migrants invaded the tracks in an attempt to climb aboard trains heading for London.

Multiple people have died on the tracks, with British Prime Minister David Cameron describing the situation as a “swarm of people” and some colleagues in the Conservative party accusing French authorities of losing control.