The review is in the early stages, with the Centrica-owned company planning to approach agencies over the coming weeks.
Carat, the incumbent, has been informed of the process, and has been asked to repitch for the account.
The agency has held the full British Gas media account since 2002, when it picked up the consolidated business after a pitch against ZenithOptimedia and OMD UK.
Last year, British Gas hired Publicis Groupe's Starcom MediaVest Group to handle the media planning and buying for its Business Services division, without a pitch. The agency handled media for the British Gas Energy360 launch, which was created to help businesses improve the way that they use energy.
Carat has overseen a number of award-winning campaigns during its time working with British Gas.
Last year, the agency picked up a Campaign Media Award for its work on the "good news stories" campaign, which celebrated the work carried out by British Gas engineers in the local community.
The media review will not affect the energy provider's relationships with its ad agencies, which include CHI & Partners, OgilvyOne, Ogilvy Interactive and Crayon.
Earlier this year, CHI created a national ad campaign promoting British Gas as the "cheapest standard gas and electricity" supplier in the UK. Work included TV, outdoor and print, as well as a tie-in with Absolute Radio.
The campaign followed an announcement by the company that it was dropping its prices by 7 per cent, the third time that it had cut them in 12 months. The price cut is expected to benefit as many as eight million households, with the average gas customer saving £55 per year.
In reaction to the announcement, the rival energy provider E.ON launched its own press campaign challenging British Gas' claims.