LONDON — Britain’s antitrust watchdog could force new rules on Apple’s and Google’s mobile platforms to open up competition.

The Competition and Markets Authority (CMA) published a draft decision Wednesday to designate Google and Apple as having “strategic market status” in mobile under its new digital markets powers.

That means it believes the companies have “substantial and entrenched” market power through their app stores, mobile browsers, browser engines and operating systems. If the designation is confirmed later this year then new rules called Conduct Requirements can be placed on Apple and Google.

Apple has warned the CMA to avoid any interventions which it claims could impact its customers’ privacy and security. It believes similar moves by the European Commission to enforce interoperability have undermined security and it is appealing a €500 million fine for failure to comply with the EU’s Digital Markets Act.

An Apple spokesperson said: “We’re concerned the rules the U.K. is now considering would undermine the privacy and security protections that our users have come to expect, hamper our ability to innovate, and force us to give away our technology for free to foreign competitors. We will continue to engage with the regulator to make sure they fully understand these risks.”

Oliver Bethell, director of competition at Google, described the CMA decision as “disappointing and unwarranted.”

“Android is open source and Chrome is built on our browser engine Blink, which is also open source. Together with Play, these offerings enable great choice, security and innovation for users,” he said, adding: “We estimate that Android has saved developers over one million days they would otherwise spend adapting to different operating models for each smartphone — the equivalent of £300 million in reduced costs.”

The CMA argues the two companies have a duopoly on mobile platforms in the U.K. and expressed concerns about: commissions charged on in-app purchases, processes around app reviews and rankings, and the firms’ favoring their own services.

CMA chief executive Sarah Cardell said: “Our investigation so far has identified opportunities for more innovation and choice.

“The targeted and proportionate actions we have set out today would enable U.K. app developers to remain at the forefront of global innovation while ensuring U.K. consumers receive a world-class experience. Time is of the essence: as competition agencies and courts globally take action in these markets, it’s essential the U.K. doesn’t fall behind.”

The consultations on the companies’ respective designations close on Aug. 20. The CMA has until Oct. 22 to make a final decision.

The U.K. investigation into Apple and Google is the CMA’s second probe under its digital markets powers which came into force in January.