The U.K.'s competition watchdog has launched an investigation into Facebook and Google over their dominance in digital advertising.

The Competition and Markets Authority (CMA) said on Wednesday it was assessing whether online platforms are harming consumers and competition in the U.K.'s digital ad market. It singled out Google and Facebook, which combined take an estimated 61% share of digital advertising revenues in the country.

The investigation marks another step toward stricter supervision of big tech companies facing scrutiny by regulators around the world over their size and influence on consumers and their data online.