The review was sparked by its media agency, Manning Gottlieb OMD, winning the £10 million media planning and buying account for rival group EDF Energy. EDF owns Seeboard, SWEB and London Electricity.
Manning Gottlieb is said to have tried to find a solution to working with both brands. However, Npower has decided to look for another agency.
Npower is in the process of contacting agencies to pitch for the business.
It has spent £2 million in the past year on advertising, plus £2 million on the sponsorship of Channel 4's homes programming and additional amounts on Channel 4's cricket coverage.
Manning Gottlieb won the EDF business through its sister agency, MBA, which was recently hired to handle EDF's creative.
The two agencies will work closely together on the account, as EDF rebrands its companies under the "E Energy" umbrella. They will also use the TBWA\Connections planning resource recently launched as a joint venture between Manning Gottlieb and TBWA.
Manning Gottlieb refused to comment on the story as Campaign went to press.