Yahoo! Inc., responding to Microsoft Corp.’s weekend threat of a proxy fight that could result in a lower takeover price, said Microsoft’s $44.6 billion offer must be increased before any merger can take place, Bloomberg reported.

“We will not allow you or anyone else to acquire the company for anything less than its full value,” Yahoo said today in a letter to Microsoft CEO Steve Ballmer. On April 5, Ballmer gave Yahoo three weeks to reach a deal with his company or face a proxy battle.

Yahoo said it is not against a combination with Microsoft at a superior price and rejected Ballmer’s charge that its business has deteriorated. Yahoo called Microsoft’s threat of a proxy fight “counterproductive” and said it is still evaluating alternatives to the bid, which was 62 percent higher than its stock price at the time of the offer.

The stock of Yahoo, based in Sunnyvale, Calif., fell 47 cents to $27.89 in early trading. Redmond, Wash.-based Microsoft rose 28 cents to $29.44.

Microsoft made the $31-a-share offer on Jan. 31. Its shares have fallen since the bid, closing last week at $29.16, which reduces the value of the half-cash, half- stock proposal to about $29.36 a share. Yahoo rejected the bid on Feb. 11, saying the price didn’t reflect its value.