Britain on Wednesday told Apple and Google to be fairer in how they distribute apps on their mobile platforms, setting out possible interventions as it plans to designate the U.S. tech companies with strategic market status over their duopoly.

The U.K.’s competition watchdog said it is proposing to impose guardrails around how tech giants Apple and Alphabet’s Google run their mobile ecosystems under a new law governing business conduct for Big Tech.

Apple Inc. and Google’s Android were earmarked by the UK’s antitrust watchdog for a raft of new rules targeting their power in mobile ecosystems, a market almost completely dominated by the US giants.