Thousands of Eurostar passengers were blocked for around an hour and half on Monday afternoon after there were technical problems with the overhead cables.
Trains have been delayed by up to 80 minutes, according to some passengers.
Traffic was reportedly on the move again around 4pm but delays were expected to continue, Eurostar said.
EuroTunnel spokesperson John Keefe said that technicians were currently in the affected tunnel, and that passengers could expect delays of up to two hours.
“The trains will be back to normal as soon as we can sort it out, but this is a technical problem, and it's just something we have to live with. We are managing people as quickly as we can, but passengers will have to bear with us until the problem is fixed,” he told The Local.
He added that with one of the two tunnels closed off, the trains were operating on a single line, meaning trains coming from both directions were using the same tracks.
He estimated that the trains would be running as normal later this afternoon after the issues were fixed and the trains configured to schedule once again.
Eurotunnel took to Twitter on Monday afternoon to say that queues to check in at the Calais station could be up to three hours long due to the additional security check waiting times as well as the “problem with overhead tunnel power supply”.
Passengers who face delays of 60 minutes or longer are eligle for compensation.