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 amounting to 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 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, which determined its fee using 47.5 million monthly active recipients of a service which differs from the 83 million monthly visitors used to designate the company under another tech legislation known as the Digital Markets Act.

It said this number is not valid as it includes visitors to its retail business which is not covered by the DSA. Zalando said 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.

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)