The U.S. government will hit the $16.4 trillion federal debt limit on Monday and turn to “extraordinary measures” to continue borrowing, the Treasury Department said, beginning a countdown until Congress either passes legislation to allow for more borrowing or the government defaults on its debt.

Treasury Secretary Timothy F. Geithner said in a letter to senior lawmakers that the Treasury would shortly begin to undertake “extraordinary measures” authorized by law to temporarily postpone the date that the U.S. would otherwise default on its legal obligations.

Geithner said the measures could create about $200 billion in “headroom” under the debt limit, giving Congress approximately two months before it must raise the debt limit.

To read Geithner’s letter in its entirety, click here.