The ex-soap actress, who played Zoe Tate in the Yorkshire Dales ITV series for 16 years from 1989 to 2005, and her partner Jez Hughes are raising money to send her for treatment in Germany.
They wrote about their devastating news on a page they set up on donations website gofundme.com.
The mother-of-two, known to her family as Ali, has been working as a yoga instructor in recent years but is well-known for her Emmerdale storylines.
These included raising awareness of LGBT issues after her character was cast as a lesbian and later her portrayal of suffering a schizophrenic breakdown.
In her fundraising appeal, Bracknell wrote: “Just over 5 weeks ago, I was feeling perfectly well, fit and healthy. Teaching yoga classes and workshops, it was a time of new beginnings: preparing to move house, youngest child fleeing the nest, and excited to be starting rehearsals for a comedy play, being back onstage, going on tour.
“But, it turns out that the universe had other plans. Life was about to be unexpectedly turned on its head.
“I began to feel breathless climbing stairs ... I just put it down to a bit of stress. My abdomen suddenly ballooned - and within a matter of a few days I looked heavily pregnant. I could barely walk or breathe. Then, one Saturday night at the beginning of September I ended up in A&E.”
The actress said that she had had an emergency procedure to remove excess fluid from around her heart.
She went on: “However, the bad news is that I have been diagnosed with lung cancer, stage 4. In their opinion, that means it’s terminal, not curable, not operable.
“A fairly brutal and bleak diagnosis but one I am determined to challenge and see from the perspective of “a glass half full”, going against a lifetime of pessimism, negativity and fear!!!!
“By the way, please imagine me writing this and speaking in quite an upbeat and cheerful voice. Do NOT imagine me sitting here writing through a veil of tears. I am not. Or feeling sorry for myself. I am not (yet).”
She wrote: “My priority is getting well and strong, so that I can continue to endeavour to be a good, mother, daughter, wife, or at least the best I can be.
“My priority is to defy expectation. My priority is to love, to laugh, and, as Bob (Dylan) said, to ‘keep on keepin’ on’.”
Bracknell’s partner Hughes explained more about what the funds were for.
He wrote: “Having done a lot of research, we have found there are incredible breakthroughs being made in the field of immunotherapy and integrative medicine, which are seeing previously ‘incurable’ cancers going into complete remission.
“A clinic in Germany called the Hallwang clinic is leading the way in this. The downside is that these cutting edge treatments aren’t yet available on the NHS outside of clinical trials and are very expensive.
“Due to difficult personal circumstances, Ali/Leah has been unable to work in her chosen profession of acting for a number of years which has seen a big drain on her resources. Understandably, she is unable to continue acting or teaching right now. This is why we are fundraising here.”
The gofundme appeal has currently raised £3,875 of its £50,000 target.