Jane Austen scholar Dr Paula Byrne claims to have discovered a lost portrait of the author which, far from depicting a grumpy spinster, shows a writer at the height of her powers and a woman comfortable in her own skin.

The only accepted portraits of Austen to date are her sister Cassandra's 1810 sketch, in which she looks cross, and an 1870 adaptation of that picture. But when Byrne, biographer of Evelyn Waugh and Mary "Perdita" Robinson and with an Austen biography due out in 2013, was given a portrait of a female author acquired by her husband, Shakespeare scholar Jonathan Bate, at auction, she was immediately struck by the possibility that it could be a lost drawing of Austen.

The portrait drawing, in graphite on vellum, had been in a private collection for years, and was being auctioned as an "imaginary portrait" of Austen, with "Miss Jane Austin" written on the back. "When my husband bought it he thought it was a reasonable portrait of a nice lady writer, but I instantly had a visceral reaction to it. I thought it looks like her family. I recognised the Austen nose, to be honest, I thought it was so striking, so familiar," Byrne told the Guardian. "The idea that it was an imaginary portrait – that seemed to me to be a crazy theory. That genre doesn't exist, and this looks too specific, too like the rest of her family, to have been drawn from imagination."

Byrne pointed out that Austen did not become famous until 1870, 50 years after her death, and the portrait has been dated to the early 19th century, around 1815, on the basis of the subject's clothes. "Why would someone have wanted to draw her from their imagination, when she was not popular at that time?" she asked.

She approached the BBC, and together they put together a documentary on the portrait, working with various experts including art historians, fashion experts and forensic analysts on the picture's background. "We approached it with an open mind," said Byrne. "We tried to cover all leads, and in the end we put our findings to three top Jane Austen scholars, and two out of three thought it was her." The scholars were Professor Kathryn Sutherland from Oxford University, Professor Claudia Johnson from Princeton and Austen expert Deirdre Le Faye. Sutherland and Johnson both agreed the picture was Austen; Le Faye did not. "She thinks it is an imaginary portrait. I did try so hard to find one single example of an imaginary portrait, but nobody could find one – they just don't exist," said Byrne. "But it's great to have the debate – it opens up a very interesting question about who Jane Austen was and who we want her to be."

If, as Byrne believes it is, the portrait is indeed Austen, then it shows a "very, very different" version of the writer than she has been seen as in the past, she said.

"The previous portrait is a very sentimentalised Victorian view of 'Aunt Jane', someone who played spillikins, who just lurked in the shadows with her scribbling. But it seems to me that it's very clear from her letters that Jane Austen took great pride in her writing, that she was desperate to be taken seriously," said Byrne. "This new picture first roots her in a London setting – by Westminster Abbey. And second, it presents her as a professional woman writer; there are pens on the table, a sheaf of paper. She seems to be a woman very confident in her own skin, very happy to be presented as a professional woman writer and a novelist, which does fly in the face of the cutesy, heritage spinster view."

The documentary, Jane Austen: The Unseen Portrait?, is due to air on BBC2 on Boxing Day.