Lefroy, who later married an heiress, named his eldest daughter Jane
A watercolour of the man believed to have inspired Jane Austen's Pride and Prejudice character Mr Darcy is expected to be sold for £50,000.
The miniature portrait of Thomas Lefroy was painted on ivory in 1798 by English artist George Engleheart.
The couple's relationship was portrayed in last year's film Becoming Jane, with Anne Hathaway and James McAvoy.
The picture will be sold at the Grosvenor House Art and Antiques Fair in London later this month.
Several locks of hair are located in a panel at the back of the miniature, although it is not known to whom they belonged.
Austen met Lefroy - who described his feelings for her as a "boyish love" - in 1796 at the age of 21.
They are said to have talked, danced and flirted, and later she called him "a gentlemanlike, good-looking, pleasant young man".
He had one fault, however, "which time will, I trust, entirely remove", she said. "It is that his morning coat is a great deal too light."
However Irish-born Lefroy married an heiress, in line with his family's expectation that he would find a wealthy wife to improve their financial position.
Austen wrote of her sadness, saying: "At length the day is come on which I am to flirt my last with Tom Lefroy... my tears flow as I write at the melancholy idea."