Slack has filed a complaint against Microsoft with the EU, claiming the company is engaging in anti-competitive practices with Teams.

Slack and Teams have been locked in a bitter battle over the corporate messaging market. While Microsoft’s app has far surpassed Slack, in terms of users, Slack has continued to rack up some impressive contracts. In particular, Slack has become a popular choice among companies that compete with Microsoft and don’t want to rely on one of their competitors for their communication.

Now Slack has upped the ante even more, filing a complaint with the EU. In particular, Slack is claiming that Microsoft is unfairly tying Teams to Office.

“We’re confident that we win on the merits of our product, but we can’t ignore illegal behavior that deprives customers of access to the tools and solutions they want,” said Jonathan Prince, Vice President of Communications and Policy at Slack. “Slack threatens Microsoft’s hold on business email, the cornerstone of Office, which means Slack threatens Microsoft’s lock on enterprise software.”

Microsoft should be concerned by this complaint, as it is similar to the complaint that was successfully used in Microsoft’s antitrust case in 2001. To make matters worse for the company, the EU is currently scrutinizing numerous US companies for anti-competitive practices. All of this means that Microsoft may find itself in an unfavorable climate should Slack’s complaint move forward.