Cisco has said it will challenge Microsoft's $8.5bn (£5.4bn) takeover of Skype at the EU's top court.
The networking giant said conditions needed to be set to ensure Microsoft would not block other video services.
In October, the European Union had ruled the deal would not impede competition.
But Cisco has called on the European Commission to introduce open standards similar to those used for mobile phones.
"Cisco does not oppose the merger, but believes the European Commission should have placed conditions that would ensure greater standards-based interoperability," Cisco's video conferencing head Martin De Beer wrote in a blog post.
European Commission spokesman Antoine Colombani said: "We will defend our decision in court."
Skype has over 650 million users worldwide.
Its takeover by Microsoft, announced in May last year, is the software giant's single biggest acquisition.
Cisco offers its own video conferencing service called WebEx. Open standards for video services would potentially allow calls to be made between WebEx and Skype, as well as other services like Google Video.
Without such a standard, Mr De Beer said Microsoft could control "the future of video communications".
"Making a video-to-video call should be as easy as dialling a phone number,'' he argued.
"Today, however, you can't make seamless video calls from one platform to another, much to the frustration of consumers and business users alike."