Slack alleged that Microsoft has "illegally tied its Teams" product to Office productivity suite and is "force installing it for millions, blocking its removal, and hiding the true cost to enterprise customers."
"We're confident that we win on the merits of our product, but we can't ignore illegal behaviour 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."
The European Commission will review the complaint and decide whether to open a formal investigation into Microsoft's anti-competitive practices.
Prince said that Slack offers an open, flexible approach that compounds the threat to Microsoft because it is a gateway to innovative, best-in-class technology that competes with the rest of Microsoft's stack and gives customers the freedom to build solutions that meet their needs.
"We want to be the 2 per cent of your software budget that makes the other 98 per cent more valuable; they want 100 per cent of your budget every time," Prince said.
Microsoft was yet to comment on the Slack complaint.
David Schellhase, General Counsel at Slack, said that "Microsoft is reverting to past behaviour.
"They created a weak, copycat product and tied it to their dominant Office product, force installing it and blocking its removal, a carbon copy of their illegal behaviour during the browser wars," he alleged.
Slack is asking the European Commission to take swift action to ensure Microsoft cannot continue to illegally leverage its power from one market to another by bundling or tying products.
Slack last month joined Amazon in a multi-year agreement to take on its closest rival Microsoft Teams. Under the new deal, all Amazon employees will get access to Slacks office tools.
After claiming that Microsoft Teams is not a real competitor to Slack, Stewart Butterfield, CEO of the enterprise messaging platform, said in May that the success of Slack threatens Microsoft's entire Office 365 empire.