Yahoo said Microsoft's original offer undervalued the firm
Computer software giant Microsoft has given Yahoo a three-week deadline to respond to its offer to buy out the internet company for $44.6bn (£22.3bn).
Microsoft CEO Steve Ballmer said his company would take its case directly to Yahoo's shareholders if Yahoo's directors did not respond by 26 April.
Microsoft made an unsolicited bid on 31 January, but Yahoo's board rejected it as substantially underrating its value. Yahoo has since explored alliances with other firms, but no offer has surfaced.
In a letter, Mr Ballmer acknowledged that such negotiations were underway, but wondered why Yahoo was not talking to Microsoft too.
"This is despite the fact that our proposal is the only alternative put forward that offers your shareholders full and fair value for their shares," he wrote.
"During these two months of inactivity, the internet has continued to march on, while the public equity markets and overall economic conditions have weakened considerably," he added.
Mr Ballmer said his company's offer - 62% above Yahoo's market value at the time - had grown stronger as time had passed.
"We believe that the majority of your shareholders share this assessment," he wrote, adding that Microsoft would take its case directly to them and work to elect a new board of directors if they did not respond within three weeks.
Last month, Yahoo estimated it would almost double its operating cash flow over the next three years and generate $8.8bn in revenue after costs in 2010.
"Yahoo provides meaningful strategic value and warrants a significant acquisition premium above its equity value in a potential change of control transaction," it said.