Rail services between Britain and France were suspended after migrants clambered onto the Eurostar track outside Calais.
In all five trains had to be held while police cleared the migrants from the track.
The invasion was the latest in a series of disruptions to the cross channel rail link in recent months.
Attempts to tackle the problem, including erecting miles of razor wire and a new 13-feet high fence have proved ineffective
Photo: Yui Mok/PA
There were also reports that the migrants had climbed on top of the trains.
Eurostar said that the trains were being sent back to their departure stations until the problem had been dealt with.
A company spokesman said it was unclear how long delays would last.
“Because we don’t know when the site will be cleared we really can’t say anything about how long the tunnel delay will be. It’s impossible for us to say right now,” he said.
Passengers turned to social media to describe the scene.
“The trespassers are on the roof of the train in front of us,” Simon Gentry said.
In June the disruption hit ferry traffic as well, with the port of Calais being shut because of industrial action.