Microsoft has been hit with an antitrust complaint regarding its cloud business in the EU, as rivals try to compete against the second-largest cloud provider.

Microsoft Azure is second only to Amazon’s AWS in the cloud market. One significant advantage Microsoft has over all of its rivals, both large and small, is the ecosystem the company has built up for decades. The business world runs on Microsoft software, including Windows and Office, and that familiarity with the ecosystem gives the company a significant competitive advantage.

According to Reuters, three of Microsoft’s EU competitors, including OVHcloud, have filed an antitrust complaint.

“Through abusing its dominant position, Microsoft undermines fair competition and limits consumer choice in the cloud computing services market,” OVHcloud said.

“We’re continuously evaluating how we can best support partners and make Microsoft software available to customers across all environments, including those of other cloud providers,” a spokesperson for Microsoft told Reuters in response.

It’s always unusual when it’s the runner-up, and not the market leader, accused of antitrust violations. According to The Wall Street Journal, however, it does appear it’s Microsoft’s bundling of its productivity software that is at the heart of the issue. OVHcloud and its co-complainants allege that it costs Microsoft Office customers more if they choose to use a third-party cloud provider, rather than Microsoft’s offerings.

The EU is generally tougher on companies than the US, more aggressively protecting consumer rights. If there is merit to the complaint, Microsoft could have a major issue on its hands.