Three major technology companies — Microsoft, Yaho and AOL — have reportedly come together to form an unusual partnership in which the rivals will sell ad inventory on each others’ sites.

All Things Digital (ATD) reported that the companies, who have traditionally competed with each other over advertising revenue, will join hands to fight one common enemy – Google.

Google dominates the search advertising market. In May, Google VP Neal Mohan said that he expects online display advertising to grow ten-fold to a $200bn+ industry by 2015.

According to eMarketer, Google dominates the search advertising market, with nearly 76% market share.

Recently, comScore said that Google Sites led explicit core search market in August with 64.8% share. In its monthly comScore qSearch analysis of the US search marketplace, comScore said that Yahoo Sites with 16.3% (up 0.2 percentage points) and Microsoft Sites with 14.7% (up 0.3 percentage points) were the other leaders in the explicit core serach market last month. comScore added that Ask Network accounted for 3% of explicit core searches (up 0.1 percentage points), followed by AOL with 1.3%.

ATD said that the companies discussed the new strategy at a private meeting in Manhattan on Tuesday night.

Officials from the three companies and from the advertising industry attended the meeting, said ATD, citing an agency executive who attended the meeting.

The three companies have not commented on the matter so far.