Apple and Google's app stores, operating systems and browsers will be investigated by Britain's competition regulator over concerns they have too much power.
The Competition and Markets Authority (CMA) said it will study the US tech giants and their services to assess whether their size means competition is being stifled.
The two firms have an effective duopoly across various key areas, with Apple's iOS and Google's Android being the world's two dominant mobile operating systems.
The Apple App Store and Google Play Store are the main gateways to downloading apps, while Google Chrome and Apple Safari are the two most used web browsers.
The CMA, which is based in London's Canary Wharf, said it will look into whether the companies' level of control is harming competition across a range of digital markets.
The app stores, operating systems and web browsers of Apple and Google are to be studied by the UK competition regulator over fears they have too much power and harm other businesses
The regulator admitted it was concerned the current status of the market could lead to reduced innovation in the sector and higher prices for consumers.
The 'market study' can make recommendations to Government and other bodies and issue guidance to businesses and consumers as needed.
Competition and Markets Authority boss Andrea Coscelli (pictured) said the watchdog's 'ongoing work into big tech has already uncovered some worrying trends'
CMA chief executive Andrea Coscelli said today: 'Apple and Google control the major gateways through which people download apps or browse the web on their mobiles - whether they want to shop, play games, stream music or watch TV.
'We're looking into whether this could be creating problems for consumers and the businesses that want to reach people through their phones.
'Our ongoing work into big tech has already uncovered some worrying trends and we know consumers and businesses could be harmed if they go unchecked.
'That's why we're pressing on with launching this study now, while we are setting up the new Digital Markets Unit, so we can hit the ground running by using the results of this work to shape future plans.'
The market dominance of the two companies, which are both based in California, has come under increased scrutiny in recent months, most notably led by complaints from Fortnite maker Epic Games.
The Competition and Markets Authority is based in this building in London's Canary Wharf
Epic has pursued legal action in the US and Europe against both tech giants over what it claims is an unfair control of the payment systems used in online app stores.
Both Apple and Google take a commission from in-app purchases carried out via their stores.
The CMA said its new study would be broader than some of the other competition probes it already has into Apple's App Store and Google's 'Privacy Sandbox'.
The regulator is already investigating Apple over its control of the App Store and developer access to it.
It is also looking into Google's plans around changes in Chrome - specifically, a new way to target advertising that the CMA fears may harm the advertising market.