BRUSSELS, April 17 (Reuters) - German online fashion retailer Zalando has challenged a supervisory fee aimed at covering EU regulators' costs of monitoring compliance with new European Union tech rules, the third company to take EU tech enforcers to court on the matter.

The Digital Services Act (DSA) obliges 20 very large online platforms and two very large online search engines to pay the annual charge capped at 0.05% of their annual worldwide net income.

The size of the annual fee is related to the number of average monthly active users for each company, the higher the number of users the higher the fee, and whether it posts a profit or loss in the preceding financial year.

Zalando said it disagreed with the European Commission's fee calculation methodology and wants more transparency on this.

It said the regulator had determined its fee based on 47.5 million monthly active users while it designated Zalando as a very large online platform based on 83 million monthly visitors.

It had twice asked for clarity from the Commission but both requests were denied.

"While with this claim we are not contesting the amount of the fee itself, we believe that without clear and transparent information on the calculation method used, we cannot verify its accuracy or fairness," its general counsel Lena Wallenhorst said in a statement.

The Commission defended its methodology.

"When it comes to Zalando's access to document request, the Commission shared in due time with Zalando the datasets underpinning the calculation of the fee to be paid by Zalando," a spokesperson said.

Meta Platforms and TikTok have also challenged the supervisory fee.

Zalando last year sued the Commission for labelling it as a very large online platform subject to the DSA. (Reporting by Foo Yun Chee Editing by Alexandra Hudson and Nick Zieminski)