Google has sent Microsoft a cease and desist letter, demanding that the YouTube app for Windows Phone be removed from the Windows Phone Store by May 22nd. The letter alleges that the app, unveiled just over a week ago, blocks advertising and allows local downloads of content, both clear violations of YouTube’s terms of service. The application was authored by Microsoft, without consultation or collaboration with Google.

The letter, obtained by and published on The Verge, further alleges that the application plays videos which content creators have restricted from playing on mobile devices, often because of agreements with third parties. The letter also claims that the app fails to follow YouTube’s branding guidelines, using the YouTube logo in a way that is likely to leave users confused about who authored the app.

Google insists that these violations harm YouTube content creators, saying in part:

“Content creators make money on YouTube by monetizing their content through advertising. Unfortunately, by blocking advertising and allowing downloads of videos, your application cuts off a valuable ongoing revenue source for creators, and causes harm to the thriving content ecosystem on YouTube.”

“We’d be more than happy to include advertising but need Google to provide us access to the necessary APIs,” a Microsoft spokesperson said in response to Google’s letter.

The spokesperson referenced comments made by Google CEO and co-founder Larry Page at yesterday’s Google I/O conference, in which Page argued that technology should not be tied to specific platforms: “In light of Larry Page’s comments today calling for more interoperability and less negativity, we look forward to solving this matter together for our mutual customers.”

This dust-up is one of a series of contentious moments between Microsoft and Google, as the two companies have tangled over a number of recent interoperability issues. The news also comes as YouTube begins expanding the scope of its offerings and its potential revenues, last week announcing a pilot program for paid subscriptions to certain channels, and next week hosting it’s first “Comedy Week,” a series of programs from well-known performers including Andy Samberg and Sarah Silverman.