Andrea Coscelli, the head of the Competition and Markets Authority (CMA), has said that Google and Facebook have too much of a share of the online advertising market in the UK.
Coscelli told the BBC that the CMA would like market dominance to be dealt with via regulatory changes.
Both Facebook and Google have been criticised in the past by other regulators, as well as their competitors.
However, Facebook claims that there is "significant competition" between rival firms in the online advertising market.
Speaking to the BBC’s Amol Rajan, Coscelli said that the two companies have a “duopoly” when it comes to online advertising in the UK, which is usually bad for competition.
The CMA boss said that it was “not an ideal situation” that Facebook and Google have an 80% share of the £14bn online advertising market in the UK.
"We think it would be good if we got to a situation where others had a bigger share of the market," Coscelli said.
He also said it was a “problem” that Google alone holds around 90% of the UK’s search advertising market, with around £7.3bn in total.
Coscelli also said that Facebook’s current 50% share of the UK’s £5.5bn display advertising market was too much.
"When companies have too much economic power, that creates a number of distortions, first for competitors, secondly for consumers, and at some level potentially in terms of the political process as well, in some cases," Coscelli said.
"We, in general terms, like to see markets more competitive, with more players, with more diversity of players, because we think that delivers better outcomes."
A spokesperson for Facebook said that the platform "gives millions of people and businesses in the UK the opportunity to connect and share."
Adding: "Advertisers can and do freely move their [advertising] spending between TV, radio, print, outdoor and online.
"And in online advertising itself, we face significant competition from the likes of Google, Apple, Snap, Twitter and Amazon, as well as new entrants like TikTok, which keeps us on our toes."
Despite having concerns about the lack of competition, Coscelli did not call for the companies to be broken up: "Our current proposal is not to break them up, it's to have pro-competitive regulation to deal with some of the issues, but it would allow the companies to maintain all the current activities that they have”.
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