Eurostar services to and from the French capital were severely disrupted on Friday with the company advising passengers not to travel.
"Due to an overhead power issue in Paris, our traffic to and from Paris is severely disrupted," Eurostar said in a statement, adding that passengers should only proceed with their travel plans only if essential.
Four services — two departing from Paris and the others from London — were cancelled.
A Eurostar spokesman told Reuters that there had been a power cable issue on one of the platforms in Gare du Nord, Eurostar's Paris base. It was not immediately clear what caused the failure.
Services from Brussels were also disrupted on Friday with delays of between 30 minutes to an hour because of speed restrictions imposed due to high temperatures.
In London's St Pancras station, social media user Charlott Gracias reported "absolute chaos" near the international departures because of the delays.
Emma Finamore, who was scheduled to return to the UK on Friday from her honeymoon in Paris, said the company is "not being much help" with rebooking tickets and that having already checked out of their hotel, they "have no idea whether to go to the station or not!"
Passengers already on board trains are also being impacted with one social media users saying passengers are "freezing to death on a stationary #Eurostar train at Ebbsfleet. Air conditioning cranked to 11."
The 14h CEST train out of London was the last Eurostar to be cancelled but severe delays continued throughout the afternoon. Services seemingly returned to normal at around 20:00 CET.
Other train companies were also impacted across Europe. Thalys, which runs international services between France, Belgium, the Netherlands and Germany, and the French railway company SNCF, had their infrastructure challenged by the heatwave.
Both companies said on Twitter that they expected a return to normal over the weekend.
Extreme heat also caused rail delays in the UK.