Mary Poppins's real-life model appears in unseen PL Travers story

PL Travers wrote about the eccentric great aunt who inspired Mary Poppins in a story which she gave away as a Christmas gift in 1941, and which is being released to the general public for the first time this November.

Travers called her great aunt Ellie, whose real name was Helen Morehead, Christina Saraset, or Sass for short. In her semi-autobiographical story Aunt Sass, she writes of how "her remarkableness lay in the extraordinary and, to me, enchanting discrepancy between her external behaviour and her inner self", adding "imagine a bulldog whose ferocious exterior covers a heart tender to the point of sentimentality and you have Christina Saraset".

The resemblance to Travers' most famous creation, the nanny whose spoonful of sugar made the medicine go down for the Banks children, is no coincidence. Travers goes on to write in the previously unpublished story about the moment she heard of her relative's death. "I thought to myself, 'Some day, in spite of her, I shall commit the disrespectful vulgarity of putting Aunt Sass in a book.' And then it occurred to me that this had already been done, though unconsciously and without intent. We write more than we know we are writing. We do not guess at the roots that made our fruit. I suddenly realised that there is a book through which Aunt Sass, stern and tender, secret and proud, anonymous and loving, stalks with her silent feet," wrote the author. "You will find her occasionally in the pages of Mary Poppins."

Travers printed just 500 copies of Aunt Sass, handing them out to family and friends. Virago will release the story to a wider readership for the first time through its Modern Classics imprint this Christmas, in an edition that will also include two other stories only given out by the author as gifts: Ah Wong, which she wrote in 1943, and Johnny Delaney, from 1944. Each original edition of the stories bore the line that it has been "limited to five hundred copies privately printed for the friends of the author as a Christmas greeting".

Ah Wong tells of the narrator's encounter with a swearing Chinese cook, and Johnny Delaney of her run-in with a jockey with a bad temper. "Each of the characters she focuses on are really unlikely heroes for the narrator. The encounters take place which she is a young girl – the stories are in the form of a person looking back over their childhood, growing up in Australia on a sugar plantation, when her parents don't seem to be very close to her, but the other characters are," said editor Donna Coonan. "And just like Mary Poppins, each character pops up just when they are needed."

Coonan said she had decided to find out if there was any unpublished work by Travers after seeing Victoria Coren Mitchell's documentary about the author, and discovered the three gift stories. "I wasn't very hopeful," said Coonan. "I thought if they hadn't been in print before, they couldn't be that good – but I was absolutely bowled over by them, moved to tears. They're incredible."

It is unclear how autobiographical the stories are; Travers guarded details about her private life fiercely. "She kind of merged truth and fiction," said Coonan. "So nobody has been able to tell which parts are real and which are not … but the stories certainly have autobiographical elements."

Travers was born in Queensland in 1899, working as a secretary, dancer, actor and journalist, settling down to write Mary Poppins in 1934 while convalescing after a serious illness. She said she wrote the book "to while away the days, but also to put down something that had been in my mind for a long time". Travers received an OBE in 1977. She died in 1996, aged 96.