An online competition entitled 'The Eurostar quest', intended to find 'the world's best code-cracker', begins this week and runs until the end of the month.
It will be backed by a TV, press and ambient campaign by TBWA\London, intended to drive consumers to the competition website.
The activity, which will run across the US, UK, France, Belgium, Australia and New Zealand, will set entrants a series of 16 challenging puzzles to solve. Five winners will then be invited to a grand final at The Ritz hotel in Paris.
Eurostar believes the activity will boost passenger numbers on its services by 10%.
The winner of the competition will receive EUR200,000 (£137,000), free travel on Eurostar for life and accommodation at The Ritz, Paris, and Claridge's, London for five years.