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Eurostar customers will need a Facebook account to access the lowest fares, the cross-Channel rail operator said.

It has launched a website named Snap which allows passengers to buy one-way tickets from just £25 as long as they log in to the social network.

The new service will only be suitable for those with a flexible itinerary as customers are not told the exact time of their train until 48 hours before travel.

Read more:17 spending secrets to find cheap hotels abroad - including for Euro 2016

Passengers can select a date and whether they want to leave before or after noon for travel between London and Paris or Brussels.

Snap comes just days after the high-speed rail operator reported a fall in passenger numbers as travellers "remain cautious" in the wake of the recent Brussels terror attacks.

There was a 3% reduction in passengers to 2.2 million in the three months to the end of March compared to a year ago, while sales fell 6% to £201 million in the same period.

Read more:Brit holidaymakers being 'ripped off' in Europe paying hundreds more than other countries

A spokesman said Snap's £25 fares are the lowest it has ever offered. Its regular website currently advertises tickets from £29.

Eurostar's commercial director, Nick Mercer, said: "Our research has shown that there is a strong appetite for spontaneous travel at great prices.

"With state-of-the-art trains boasting 20% more capacity, as well as WiFi and onboard entertainment, travellers with a flexible schedule can look forward to a spur-of-the-moment trip at the lowest fare."

Snap will initially be open until June 4 for journeys until the end of that month - the deals can be found at www.snap.eurostar.com.

After this period it will open and close depending on how many empty seats are available.

Bookings must be made at least seven days in advance. For more ways to save on travel across the continent, see our handy guide on How to get cheap Eurostar ticket deals .

Any other offers on Eurostar tickets?

Eurostar offers sales on tickets relatively often – there have been two in the last six months – where you can bag return tickets for £59.

The first thing to do if you're looking for cheap tickets, is to head to the Eurostar deals page, or the Voyages-sncf deals page for trips to southern France on the new Eurostar routes.

There are also two Eurostar loyalty programmes: Eurostar Plus Points - where you receive £20 off your next trip for every £300 you spend – and Eurostar Frequent Traveller, which offers rewards including access to business lounges, fast-track check in and deals at partner hotels.

You can check The Trainline Europe for Lyon, Lille, the Mediterranean, Bruges, the Alps and Amsterdam. For travel tips, enter your destination into TripAdvisor's search bar, or see our page on cheap train tickets.

If you're looking for a cheap deal on your whole holiday Lastminute.com has some deals including hotels here.