The Commission, which is the EU antitrust authority, in 2014 fined the German telecom firm 31 million euros for abusing its dominant position on the Slovak market for broadband communications.
In 2018, the General Court reduced the fine by 12 million euros, which the EU executive paid back to Deutsche Telekom two months later.
However Brussels refused to pay interest for the period between the payment and the partial reimbursement.
The court found this to be a "serious breach" of EU rules and ordered the Commission to pay Telekom 1.8 million euros.
(Reporting by Francesco Guarascio @fraguarascio; Editing by Kevin Liffey)