Microsoft CEO Steve Ballmer lowered the boom on Yahoo April 5, telling the company that if the two companies can't come to a decision regarding Microsoft's $31 per share purchase offer within three weeks, it will take its offer directly to Yahoo's shareholders.

Ballmer offered the ultimatum, which signals the beginning of a proxy fight, in a letter sent to Yahoo's board Saturday, an unusual move for a dialogue that has been conducted throughout the course of the business week.

The letter comes after Microsoft and Yahoo have held casual but fruitless negotiations regarding Microsoft's offer, which was worth $44.6 billion when it was first made Feb. 1, but has sunk to about $42 billion due to decline in both company's shares.

Read the full article: Ballmer Threatens Yahoo with Hostile Takeover.