Vicky Phelan says she is "so disappointed" to be missing Charlie Bird's Croagh Patrick climb on Saturday, but she will be with everyone "in spirit" on the day.
The campaigner has been involved in the Climb with Charlie fundraising effort from the get-go, but says she simply couldn't risk the journey to Westport this weekend.
"I just couldn't take the chance," she said in a video message posted to her Instagram on Thursday night.
"My walking is still quite limited and my back is still causing me quite a bit of pain.
However, the campaigner said a number of her family and friends including her parents, sister and two of her brothers as well as nieces and nephews will take part in the climb on her behalf.
Encouraging everyone to get involved in some way, the campaigner said just going for a walk is something "we all take for granted".
"And it's something I can't do at the moment. I can walk from my kitchen to my front door and that's pretty much it. Then I have to sit down.
"If it's something that you can do, don't take it for granted.
"If you can get out there on Saturday, please do. Even if it's just up and down your stairs or in your local park."
On Friday morning, Charlie Bird shared a video on Twitter with a message delivered via his newly cloned voice.
In the message, Bird confirmed that the campaign is "moving fast" to its target of raising €1m for the Irish Motor Neurone Disease Association and Pieta House.
"I think we are now going to go beyond our target," he added.
Thanks to everyone across the country and in many places around the world I think we are now going to go beyond our target of raising one million euro for our two great charities.
— Charlie Bird (@charliebird49) April 1, 2022
Everyone who is supporting climb with charlie is extending the hand of friendship. Thanks. pic.twitter.com/devGZgVo5o
The broadcaster also shared an image of five candles which he plans to light at the top of Croagh Patrick tomorrow.
The candles are dedicated to Ukraine, those working on the frontline of the Covid-19 pandemic, those who are ill, Vicky Phelan, and those "who climb their own mountain."
Bird, who was diagnosed with motor neurone disease (MND) last year, will climb Croagh Patrick this Saturday alongside hundreds of others who are scaling the Reek or other mountains across the country for the cause.
A number of familiar Irish faces will join the event including former President Mary McAleese, country singer Daniel O’Donell, Wexford hurling manager Davy Fitzgerald, CNN reporter Donie O’Sullivan, architect Dermot Bannon and RTÉ broadcaster Ryan Tubridy.
More than 100 climbs across the country will take place simultaneously as part of the fundraising drive. The proceeds from a charity single recorded by a host of Bird's former RTÉ colleagues and friends will also go towards the two charities.
Supporters can also donate to the campaign at the tills in Dunnes Stores this weekend.
You can register your own fundraiser, make a donation or find out where to join your local climb on April 2 on climbwithcharlie.ie.