Work on creating an interchange for the Eurostar at St Pancras will mean extensive engineering works on Thameslink's routes for six months from June this year.
During the same period in 2004 its through-London service will be broken up with part of the line closing. For those six months, services from the south will terminate at Kings Cross Thameslink, with those coming from the north terminating at St Pancras.
The integrated marketing and PR campaign will have a strong PR focus and is expected to wield a budget in the region of £1.5m.
The Haystack Group has been hired to handle the pitch process. Nine agencies have been seen so far with a shortlist of three expected to be finalised within weeks. An appointment is expected within the next six weeks.
The successful agency will report to communications director Martin Walter and commercial manager Peter Robinson.
Robinson said: 'With the amount of work required I can't see it being done for any less than £1.5m. This (campaign) is a measure to retain revenue - this is about getting the best information to our customers to help them plan their journeys. We need to get the message across constructively. This is not puffy PR, it is a real gritty job.'
He said that although Thameslink is looking for one 'communications agency' to handle the brief, 'we expect the key people to be PR literate'.
He emphasised that following the work Thameslink will retain its current 'USP' of running a continuous through-London service from Bedford and Luton north of the capital to Brighton and Gatwick in the south.
The engineering work this year ahead of next year's partial line closure will mainly take place at weekends, affecting visitors to London rather than regular commuters.
Robinson said there are three main elements to the campaign, handling media relations, communicating directly with passengers and an internal comms drive.
The Thameslink franchise's majority partner is the publicly listed Go-Ahead Group. The CEO of Go Ahead's rail division is Keith Ludeman.