The airport's appointment of Fishburn Hedges and sister agency Seventy Seven coincides with BAA publishing statistics revealing that Stansted has slipped from the UK's third busiest airport to fourth.
Adam Keal, head of corporate and brand reputation at Fishburn Hedges, said: 'Passenger numbers are the most significant challenge, but we are optimistic because there is a very positive story here.'
He said the agency's strategy was still in the planning stages, but added that it would promote the airport as a medium-term solution to the South East's airport capacity crisis. It will also emphasise its position as the closest major airport to the Olympic Stadium in Stratford.
'The debate is still raging about how the South East can accommodate more flights without extra runways, but the answer is staring us right in the face,' said Keal.
'Stansted is not operating at maximum capacity and has the infrastructure to handle twice its current number of passengers and jumbo jets.'
Keal said handling BAA's ongoing fight to retain ownership of the airport; Air Passenger Duty; fuel prices or plans for a second runway were not part of the brief, although it was possible that the account might 'veer off into other directions'.
Finsbury continues to handle BAA's public affairs.
Keal singled out the airport's 21st birthday, taking place this year, and London 2012 as 'licence to roam' with a more creative PR strategy. On the corporate side, the agency will be holding briefings with the business media.
BAA declined to comment on the value of the account or the other agencies that had pitched.
While other airports in the BAA portfolio posted a rise in passenger numbers last year, Stansted's fell 2.8 per cent to 18 million. This put it behind Heathrow, Gatwick and, for the first time, Manchester.
The number of passengers using the airport has been in steady decline since reaching 27 million in 2007.