The energy supplier's decision to consolidate is thought to be driven by a bid to drive down agency costs.
The incumbent on the £10 million above-the-line account, Beattie McGuinness Bungay, is not expected to pitch for the consolidated business. EHS Brann, its direct agency, and Grand Union, its digital agency, are also unlikely to feature.
BMB initially won the ad account from Isobel in August 2006 on a project basis.
The Aardman Animations characters Wallace and Gromit starred in BMB's latest ads for npower, where Wallace showered while Gromit dealt with a decrepit boiler.
Npower was unavailable for comment.