The single piece of paper - valued at up to pounds 150,000 - was handwritten by George Washington and contains the most important section of the visionary speech he intended to make at his inauguration as the first president of the United States.
Washington abandoned the speech, delivering instead a shorter, less radical version when he was sworn in on 30 April, 1789.
The original draft was broken up, with no record taken of the full text, and given away to autograph hunters.
Now, after 207 years, the kernel of Washington's vision of future American greatness, contained on pages 34 and 35 of the 62-page document, is to be auctioned in London after surfacing by chance during a routine probate valuation of books at a house in Aldeburgh, Suffolk.
A gardener showing Simon Roberts, of Phillips' book department, around the house spotted in the corner an album, covered in faded silk, protruding from under the sofa in the drawing room.
Among a vast sheaf of documents he made the amazing discovery, later confirmed as authentic by Phillips' senior book specialist, Felix Pryor, in London.
"I couldn't believe my eyes when I read the letter," Mr Pryor said. "To scholars of American history, this is the Holy Grail - what autograph collectors dream about at night.
"It is truly a great discovery, representing something of an oratorical climax. Both its style and vision entitle it to consideration as the undelivered Gettysburg Address of the American Revolution."
It is being offered at Phillips on June 13 with a pre-sale estimate of pounds 100,000 to pounds 150,000.
Mr Roberts said: "It is by far the most important find of my life."Reuse content