An agency is being sought for both Southern and the Gatwick Express brand, which Southern operates.
Southern media relations manager Chris Hudson said Gatwick Express had been using a combination of the in-house team and Surrey-based agency SRF for consumer PR for the past two years.
Southern has been using a mix of its in-house team and Good Relations.
Hudson added: ‘We will appoint an agency through to end of franchise, which is July 2015.' The contract will go live from 1 September 2012.