The power company npower has complained to the Advertising Standards Authority about recent British Gas advertisements that it says falsely suggested the company had introduced the lowest gas price rises in the market since 2002.

The newspaper ads, which appeared in the Daily Telegraph and the Guardian on July 19, were entitled "Price rises. How do they really stack up?" They showed a bar chart of pound coins and featured the gas price increases imposed by the top six power companies. They stated that British Gas had imposed the lowest gas price increases - 26% - of any of its rivals since January 2002.

In its letter to the ASA, npower claims the ads misled consumers into thinking that British Gas rises have been the lowest in both monetary and percentage terms.

Npower said the fact that British Gas's prices were the highest to start with meant its increases were some of the largest imposed. It also disputed British Gas's current TV campaign, which suggests that those switching away from the former monopoly gas supplier will no longer have access to British Gas engineers and its central heating products.

"The adverts give the impression that British Gas's prices are the lowest when, in fact, the opposite is true. According to most calculations, the average family with British Gas is paying £99 a year more than they would if they switched to the cheapest provider. These adverts wrongly suggest that the British Gas is cheaper," an npower spokesman said.

British Gas said yesterday that it stood by the adverts but confirmed that consumers who have switched suppliers could still buy its central heating policies.

"We will defend the adverts vigorously if the ASA decides it wants to examine the matter further. We are all operating in a highly competitive market and I guess the advertisements must have touched a nerve," a spokesman said.

The ASA confirmed that it had received the complaint and said it would consider whether to launch a formal investigation. A decision is expected next week.

· The ASA said yesterday it was increasingly being drawn into disputes between business rivals. In its 2004 annual report, published in April, it showed a 9% increase in such complaints - the first increase for two years. It said easyJet and Ryanair had complained to the ASA about each other, and that the telecoms companies and broadband providers were following their lead.