Users of Eurotunnel have been told to keep vigilant and check for stowaways.
The warning comes after a woman found a man hiding in her car in Calais before using the tunnel.
A Eurotunnel spokesman said while the company’s site was “extremely secure”, “everybody travelling through Calais should be vigilant whenever they leave their vehicle”.
Around 100 migrants looking to access the UK attempted to storm a P&O ferry on Wednesday and had to be held back with fire hoses.
French authorities have asked for help from their British counterparts in dealing with the migrant issue.