Yahoo and Google, the world’s two biggest search engines, had announced a two-week experiment which is to share advertising space.

During the pilot, Google will be able to place ads alongside 3% of results of Searches on Yahoo’s website.

MS criticized Yahoo’s advertising trial with Google, saying any such deal would not be in the consumers’ interests.

“Any definitive agreement between Yahoo and Google would consolidate over 90% of the search advertising market in Google’s hands. This would make the market far less competitive,” Brad Smith, Microsoft’s General Counsel said.

But Yahoo said the testing did not necessarily mean that “any further commercial relationship with Google will result”. Investors reacted positively to the announcement, with Yahoo shares rising 7%.