The oil giant, which spends £1 million above the line in the UK, has issued a request for details of costs and services that are currently provided by the WPP network.
The company has also approached between five and ten other networks for comparative information, in what it terms a benchmarking exercise.
Agency Insight is co-ordinating the process, which is being led by BP's procurement department.
A spokesman for BP insisted it was not a pitch, but said the possibility of it leading to one could not be ruled out. He said: "It is a standard information-gathering exercise aimed at improving our relationship with O&M.
"We want to make sure we are getting the optimum service with regards to our advertising and the best way of doing that is by looking at the external market."
The BP account was won by Ogilvy & Mather Worldwide in 1999. Its London office is the lead shop for the business.
Over the past few years, BP has brought all of its brands under one umbrella at the agency. This includes Castrol and its US brands, such as the service station AMPM.
Most recently, O&M unveiled a multimillion-pound integrated campaign for BP's premium fuel, BP Ultimate, which competes with Shell Optimax.
The global media account, which is held by MindShare, is not affected by the review.