With a journey time from London to Paris shrinking to just over two hours, Eurostar will attempt to convince all travellers that there is no longer any point in flying.
Director of comms Paul Charles said the account would be worth between £250,000 and £500,000.
Agencies have yet to be shortlisted, but Charles is thought to have held talks with Freud Communications, Blue Rubicon and Chime Communications.
Charles said he wanted to ensure the benefits of the move to St Pancras - which promises to reduce journey times by up to 25 minutes - were known.
The company wants to increase passenger numbers from 7.2 million last year to nine million by 2007. It plans to hire an agency by October.
Eurostar uses Media Relations Management for strategic advice and some corporate and consumer work. Its brief is unaffected.