Google and Yahoo have announced a surprise two-week team up that will see the two web giants sharing online advertising space as a limited "trial".

Microsoft, currently in negotiations to buy Yahoo, has issued an official comment on the announcement.

The software company has released the following statement from Brad Smith, Microsoft's general counsel:

"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, in sharp contrast to our own proposal to acquire Yahoo! We will assess closely all of our options."

"Our proposal remains the only alternative put forward that offers Yahoo! shareholders full and fair value for their shares, gives every shareholder a vote on the future of the company, and enhances choice for content creators, advertisers, and consumers."