Eurostar has now taken more than 15,000 extra bookings from people left stranded by the airport security alert.
Queues built up at Eurostar's Waterloo terminal
The company said it was expecting to carry 34,000 passengers on Friday, which it said was a "significant increase" compared to normal business.
The operator of the high-speed link between London and Paris and Brussels expected 23,000 passengers on Thursday but ended up 24% up on normal business.
Extra staff are helping passengers at London Waterloo and Ashford in Kent.
More call centre operators were needed as the number of inquiries to Eurostar's Ashford base doubled.
Queued for hours
People who had already queued for hours at the airports headed to Eurostar's Waterloo terminal to join long queues there instead.
Spokesman Simon Montague added: "We were due to carry 23,000 passengers during the course of Thursday.
"By lunchtime we had sold another 4,500 tickets, so we are seeing a very big increase in demand."
An extra 6,000 bookings were made for Friday.
Anyone still hoping to buy tickets is advised to book them online, to keep disruption at the south London terminal to a minimum.