Last week, Microsoft released a YouTube client for Windows Phone that gave users of Redmond's smartphone platform a rich, capable YouTube experience that didn't depend on using the YouTube Mobile site.

Though the app included account support, playlists, commenting, and most other aspects of YouTube, there's one thing it was missing—advertising. It also had two features it shouldn't have had—the ability to download videos and the ability to play videos that the creators have blocked from mobile devices.

As a result, Google sent Microsoft a cease-and-desist demand ordering the company to stop distributing the application by May 22nd.

The letter, addressed to Senior Director of Windows Phone Apps Team Todd Brix, says that "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."

In 2011, Microsoft complained to the EU that Google was barring access to certain unspecified YouTube APIs and was thereby preventing the company from properly indexing and searching YouTube videos in its Bing search engine. Microsoft also said that the restrictions prevented it from building a complete and capable YouTube application for Windows Phone.

The appearance of the application last week initially gave the appearance that the root cause of this complaint had been addressed, at least to the extent that Microsoft had sufficient access to build a decent application. The cease-and-desist, however, suggests that Redmond may not have had Mountain View's blessing after all.

Update: Microsoft has issued a response:

We'd be more than happy to include advertising but need Google to provide us access to the necessary APIs. 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.

It's perhaps notable that the YouTube app for Xbox does include support for ads, and has done since last year.