CROSS-Channel travellers are being warned to stay alert after a Eurotunnel passenger travelling back to the UK found a stowaway in the back seat of her car.

The incident followed the attempted storming of a UK-bound ferry by migrants at the French port of Calais.

Eurotunnel spokesman John Keefe said today that the company's site at Calais was extremely secure.

But he added: "Everybody travelling through Calais should be vigilant whenever they leave their vehicle."

The stowaway incident involved Sue Taber, 59, of Shepherdswell in Kent.

She said she believed the stowaway may have gained access to her car when she checked her two dogs in at the pet reception in Calais.

She said: "This boy must have been 18 to 20. I don't know how he managed to fit in the car. My dog was going berserk but I thought she was looking for a ball, and I kept feeling somebody or something - now I know somebody - nudge me in the back. I immediately called the police because I was more frightened about what he was going to do. I can't believe that it happened."

Police said a man was later detained and passed to immigration officials.

Eurotunnel and cross-Channel ferry companies said yesterday their services were operating normally.

P&O Ferries' spokesman Chris Laming said: "People need to be vigilant, but they should not be afraid. We understand the French have around 80 police officers in the Calais area and that around 40 riot police are ­travelling up from the south of France to join them."

There has been increasing anger across the Channel at the rising number of migrants flocking to Calais.

The port town has long been used as a staging post for migrants desperate to try to reach British shores, with Calais officials making direct appeals for UK help.

Calais mayor Natacha Bouchart this week stoked tensions by threatening to blockade the ferry port in an effort to force Britain to act, while Labour has also called on the UK Government "to take strong action".