© Reuters. Jennifer Bailey VP Apple Pay at Apple, speaks during an Apple special event at the Steve Jobs Theater in Cupertino

By Foo Yun Chee and Victoria Waldersee

LISBON (Reuters) - Apple Pay has attracted regulatory scrutiny as a result of several expressions of concern about the U.S. tech giant's mobile payment service, Europe's antitrust chief said on Thursday.

Margrethe Vestager's comments come after an EU questionnaire, seen by Reuters late last month, showed EU regulators had asked online merchants whether they had been told to use Apple Pay instead of competing services.

"We've been asking quite a number of questions because we get many many concerns when it comes to Apple Pay for pure competition reasons," said EU Competition Commissioner Vestager, without specifying who had expressed concern.

"People see it becomes increasingly difficult to compete in the market for easy payments," she told a news briefing at the Web Summit tech conference in Lisbon.

In an interview with Reuters last year, Vestager said she might investigate Apple Pay if there were formal complaints. At least one party has gone to the European Commission with its grievance, two people familiar with the matter told Reuters.

Apple Pay, which was launched in 2014 and is available in more than 50 countries including over 20 EU member states, marks Apple's (O: ) diversification from sales of devices like iPhones and iPads.

Apple has previously said its payment system offers the safest solution in the market, as shown by the thousands of banks using it, and that iPhones offer users a choice of payment options, including credit and debit cards and bank apps.

Vestager, who retains the competition portfolio in the new Commission due to start work on Dec. 1, has gained a reputation as a tough enforcer especially against tech companies, handing out hefty fines and ordering them to change their practices.

In the next Commission she will have an expanded mandate which could see her take a more proactive role, including introducing new legislation to govern the technology sector.