US authorities have launched a new inquiry into Google's advertising methods, amid accusations that the company breaches anti-trust laws there.
The company is by far the dominant player in internet ads, and derives most of its €40bn revenue from selling ads on the web around the world.
Now, however, the FTC is set to look at whether Google is abusing its position as the market leader. The investigation – one of many Google has had to endure in recent years – is believed to be focused on the company's acquisition of the ad firm DoubleClick in 2007.
Google's competitors are understood to have claimed that since that deal was done, Google has used some of the main DoubleClick products, such as the ad managing system, which has an estimated 80pc of the market, to push websites to use other Google products, including Ad Exchange where websites swap ads.
The new investigation will come barely four months after the FTC completed its last probe of Google.
Then the FTC was looking to see if Google had manipulated its web search results to the detriment of its rivals.
Google was the number one player in the $15bn (€12bn) US display ad market in 2012, with a 15.1pc market share, compared with Facebook's 14.6pc share, according to industry research firm eMarketer.
Google is expected to widen its lead to 20.7pc of the market in 2014.
Google is trying to convince European anti-trust investigators to wrap up a separate anti-trust probe, and has offered to change some search pages to give more space to rivals in order to satisfy their concerns.
At the Google big-tent event in London this week, CEO Eric Schmidt faced criticism over the amount of tax it paid in the UK. (Additional reporting by Reuters)