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Apple could be set for a clash with the European Commission if it's found to be flouting competition law while promoting the use of Apple Pay, its mobile payments system.

The Competition Commission circulated a questionnaire among online retailers in August asking if Apple had restricted online payments made through apps or other services, according to Reuters on Wednesday. The questionnaire reportedly said the Commission had received information that Apple may have restricted online payments made through apps or other online retailers.

Apple Pay is one of several mobile payment services on offer to phone users and is widely available across the EU. The service is deeply integrated into Apple's iPhone, which prompts users to set up Apple Pay and is the default mobile payment option on the phone.

The Commission hasn't announced a formal investigation into the service, but is clearly interested in the contractual obligations Apple is asking companies to commit to. Neither the Competition Commission nor Apple responded to request for comment.

Apple has endured previous run-ins with Europe's Competition Commission -- the two are fighting over a $14 billion tax bill issued to the company in Europe's courts. The Commission is also investigating a complaint into Apple Music instigated by rival music subscription service Spotify.