Serco group head of media and public relations Graham Capper said the company had shortlisted two agencies for the account but had already eliminated WS Square Mile from the pitch process.
'We have taken a look at our corporate PR and have decided to review all of our active communication needs,' Capper told PRWeek.
'Serco is aware that it has not punched its weight in communications and we want to look at this with a new perspective.'
WS Square Mile chair Susan Ellis said the firm would continue to work on corporate and other non-financial projects for Serco in the UK and overseas.
'Serco is looking for a new basis for financial PR and we are not tendering for that work,' Ellis said.
Capper said Serco's pitch was confined to its group financial comms brief, which is understood to be worth up to £150,000.
But he added that the company was encouraging its divisions to invest in, and review, their PR arrangements independently.
WS Square Mile has worked for Serco since 2000, though its relationship was on an ad hoc basis until the arrival of Serco director of corporate communications Dominic Cheetham from British Gas last year.
Cheetham made WS Square Mile Serco's full-time retained agency ahead of the current comms review.
In recent years Serco has benefited from government investment in infrastructure.
The company operates east London's Docklands Light Railway, and manages air traffic control centres and some schools.
Last year Serco made a fifth of its profits from running the Atomic Weapons Establishment and in March was named preferred bidder for a £1.2bn private finance initiative contract to refurbish three Leicester hospitals.
But last year Serco had to withdraw from several business segments, including UK rail infrastructure maintenance, because the activities failed to make adequate returns.
Serco made a pre-tax profit of £57.4m in 2004, up 8.6 per cent from the £52.9m it made the previous year.