Eurostar has said all services out of Paris have been suspended while firefighters deal with an incident.
There is currently no disruption to UK services, the operator said.
It comes after a lorry fire caused severe disruption over the weekend.
Eurostar passengers have been given 60 days to rebook or claim a refund on today's tickets after the suspension of train services through the Channel Tunnel.
Rail unions have launched a fresh attack over plans to sell Britain's stake in the Eurostar rail service as the government prepares to announce the next move in the bidding process.
The Rail, Maritime and Transport union said selling the UK taxpayers' 40% stake was an "act of betrayal".
The British government put the stake up for sale last December as part of a plan to privatise state assets. The French state train company SNCF owns 55% of Eurostar and Belgium's SNCB holds 5%.
Eurostar has recorded a 7% increase in sales in the first part of the year due to a surge in business travellers.
Sales revenue for the January-March period reached £227 million, a 7% increase on the year-earlier period.
Passenger numbers for the first three months of this year were up 3% to 2.3 million, with the number of business premier class travellers rising 6%.
Eurostar chief executive Nicolas Petrovic said, "The improvement we have seen in the corporate travel market in recent months underpins the strong performance reported today."
Travelers at St Pancras station have posted pictures of the scene on Twitter, with some describing the situation with the hashtag #chaos.
Eurostar told ITV News that customers have a choice of re-booking and getting compensation, or of a full refund, as well as being reimbursed for reasonable resulting accommodation costs.
Hundreds of travelers are queuing and waiting at St Pancras after severe delays meant a backlog of Eurostar services.
A fatality in the Lille region and infrastructure problems in the Eurotunnel mean few customers will be able to travel today, a spokesperson for Eurostar told ITV News.
Video footage shows the scenes at St Pancras, where some queues appear to stretch much of the length of the station.
There are "severe delays" on the Eurostar after a fatality and infrastructure problems in the Eurotunnel caused a backlog of trains.
Eurostar's twitter feed advises customers who are not currently at the station not to make their way there this evening, saying "we will be unable to accommodate you tonight".
It also says customers trying to contact them by phone could have to wait up to 20 minutes.
A fatality in the Lille region was followed by a technical hitch in the Channel Tunnel, resulting in a backlog of passengers on the tunnel's shuttles.
Eurostar say customers have a choice of re-booking their journey for free along with some compensation, or of getting a full refund for their journey.
They say they will also reimburse resulting accommodation costs.
Pierre Delalande from Eurostar said four trains in and out of England were affected, but that three of those had now reached their destination of London, while one was forced to turn back to the capital.
Trains do not run during the night and a full service is expected to resume this morning.
Mr Delalande said:
Customers were offered taxis and hotels where necessary, he added.
Eurostar has confirmed that a lightning strike close to the UK entrance of the Eurotunnel has set a building on fire. As standard procedure, Eurostar services have been stopped until the fire is brought under control. Fire services are at the scene, but the flames are not yet under control.
4 trains have been affected by the fire. 1 London to Paris train has turned around and been sent back to London. 2 Paris to London trains and a Brussels to London train are still being held at either end of the tunnel, but are said to be moving "soon". No trains are being held in the tunnel.