The move is the latest development in an acrimonious battle between Npower and British Gas.
It comes in response to two press and outdoor ads that have been run by British Gas in the past few weeks.
Npower has lodged complaints with the Advertising Standards Authority (ASA) about the ads and is understood to have held discussions with Powergen to strengthen its argument.
The first of the two ads, which uses the line 'Walk away from British Gas and wave goodbye to our engineers', has been described as inaccurate by Npower. In a letter to the ASA it claims: 'British Gas customers who are part of the Homecare scheme and yet choose to go to another supplier can still take their service agreement with them and have access to engineers.'
The second ad highlights the hotly disputed subject of price rises and is intended to dispel the myth that British Gas has imposed more increases than its rivals.
The ad, entitled 'Price rises. How do they really stack up', uses pound coins to suggest its rises have been the smallest. Npower has also disputed this, claiming: 'British Gas were and still are the most expensive of the six major gas suppliers.'
British Gas has hit back by saying the ads are fair and that it has no plans to pull the price-rise ad. A spokesman added that it would 'vigorously defend' itself against any ASA investigation.
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