Video footage has emerged showing migrants trying to break into lorries bound for Britain, a day after a strike by ferry workers caused huge traffic congestion at the port city.
Long lines of stalled vehicles remain after union members went on strike on 23 June, setting fire to tyres and blocking the entrance to the port.
In the video filmed by Reuters, groups of migrants can be seen hovering around the vehicles, with some trying to break in to the back of the trucks.
Prime Minister David Cameron called the scenes of migrants trying to board lorries during the strike "totally unacceptable", saying that both the UK and France needed to work together to tackle the problem and warning against "either side trying to point the finger of blame".
About 3,000 migrants are believed to be living around Calais, waiting for an opportunity to cross the Channel into the UK.
The UK government pledged £12m ($16.9m) last year to help France tackle the problem of illegal immigrants trying to enter Britain through Calais.
The Home Office said that about 19,000 attempts to cross the Channel have so far been thwarted.