The passenger transport company has approached a number of undisclosed agencies, and is drawing up a shortlist to pitch for the business in early December.
Poke, which recently worked on a project for the company, has declined to pitch. National Express' in-house marketing team is handling the process.
The winning agency will produce an online ad campaign to drive consumers to the company's e-commerce site. The activity will form part of a wider marketing initiative to build online ticket sales.
The ads will also be designed to encourage consumers to visit the website for National Express' new door-to-door airport transport service.
The site, www.dot2.com, created by the usability design and consultancy CAD Interactive, launched last week and offers individuals and families a pick-up service from their homes to UK airports.
The most recent online ad push for National Express, which launched this summer, was aimed at drawing in younger customers.
