The campaign was set up to pay for treatment in Germany, and has received more than 2,000 contributions within three days. The total raised on the GoFundMe website has now reached £54,580 of the £50k goal - including a £5,000 pledge from one anonymous donor.
Leah has thanked fans for their incredible generosity, writing: 'I am completely overwhelmed. I almost don't know what to say, other than I am so deeply moved and touched and humbled by your incredibly generosity and open hearts. So much kindness from old friends and new.
'I had no idea, did not expect anything like this. From the bottom of my heart I thank you, and send oceans of love your way, wherever you are and whatever you may be going through now. I feel very blessed. Very lucky to have a chance of treatment options because of you. I am in everlasting gratitude.
'Life can sure chuck some s**t at you. But when your glass is half full (as of course mine is these days!) you may be surprised to see that there is still beauty, humanity, love, kindness, hope, joy, community all around, in each and every breath.'
Leah's heartbreaking news was originally shared with fans on her GoFundMe page set up by her partner Jez, who wrote: 'Thank you for visiting this page for my beautiful partner, Ali's battle with, what doctors have labelled, terminal cancer.'
'To many she is known as Leah Bracknell, and has enjoyed a 30 year career on TV and stage, and is best remembered for portraying Zoe Tate in Emmerdale for sixteen years. To others she is a yoga teacher who has inspired thousands.'
'To us, she is Ali, wonderful mother of two, loving partner and daughter. We desperately hope that we , her friends and loved ones, can raise the funds she needs for a potentially life saving treatment.'
Leah is best known for her long-standing role in Emmerdale
Leah, whose real name is Alison, then added herself: '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.'
She describes becoming unexpectedly breathless and bloated, with symptoms worsening until she ended up being admitted to hospital, and having surgery to remove a large amount of fluid from around her heart.
'I was told later, without that, I would have died,' she says. 'So I am beyond grateful. Without it, I would not be here, enjoying the Autumn sunshine, watching the leaves change colour, settling into our new home. I would not know either quite what brilliant, generous and loving friends and family I have.'
'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).'
'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 said, to "keep on keepin' on".'
'We are looking to raise the money to visit the Hallwang clinic in Germany and help go towards the costs of Ali's treatment, to give her the best possible chances of not only survival, but to thrive,' Jez explains, and wellwishers quickly responded to the request, donating more than £17,000 in a matter of hours.
'To me she has given love, patience, kindness and shone an incredible light in my world,' he says. 'We want to make sure the light she shines radiates for a long time yet, and are all determined she can beat this.'