Google and Facebook's dominance of the online advertising market means there is a "lack of real competition" that could be limiting choice for consumers and raising prices for advertisers, Britain's competition regulator said Wednesday.

The U.K. Competition and Markets Authority (CMA) detailed a long list of concerns with the prevailing market position of the two tech giants. Among its chief complaints were that default search settings and the collection of personal data are negatively impacting on competition.

Britain's antitrust watchdog found that Google earned more than 90% of the revenues in search-based advertising in the country, totaling about £6 billion ($7.8 billion), while Facebook accounted for nearly half of all online "display" ad revenues, raking in over £2 billion.

About 15% of questions on Google "have never been searched for before," the CMA said, and rival browsers like Microsoft's Bing wouldn't have the same access to such data. That means Google enjoys a "powerful position" as the Silicon Valley giant's engorged trove of information strengthens its algorithms and makes search results more accurate.

Google's practice of paying companies to make its search engine the default option on their products has also come into question. The CMA said it found Google was willing to pay roughly £1 billion, or 16% of its search-related revenues, to be the default provider on Apple's iPhones. The regulator also questioned Facebook's targeted advertising, saying consumers are forced to share their data "as a condition for using the service."

The regulator further warned of Google and Facebook's market power restraining the revenues of newspapers and other publishers, which have complained of unexpected drops in traffic due to changes to the platforms' algorithms.

While it stopped short of making any concrete plans, the CMA said there was a "strong argument for the development of a new regulatory regime" governing digital advertising. Google said it would "continue to work constructively" with U.K. authorities, while Facebook said it was "fully committed to engaging in the consultation process around the CMA's preliminary report."