US tech titan Google is using "anti-competitive practices" with regards to advertisements on websites, Britain's competition watchdog concluded Friday in provisional findings of an investigation.

The Competition and Markets Authority "provisionally found that Google is using anti-competitive practices in open-display ad tech, which it believes could be harming thousands of UK publishers and advertisers", the CMA said in a statement.

The findings come as the US Department of Justice and European Commission carry out similar investigations into Google.

"The CMA has provisionally found that, when placing digital ads on websites, the vast majority of publishers and advertisers use Google's ad tech services in order to bid for and sell advertising space," Friday's statement added.

The UK watchdog said it "is concerned that Google is actively using its dominance in this sector to preference its own services.

"Google disadvantages competitors and prevents them competing on a level playing field to provide publishers and advertisers with a better, more competitive service that supports growth in their business."