The Irish singer, 61, was confirmed to be battling cancer alongside her sister Anne earlier this month.
Now, Nolan has offered up her support for Harding after hearing of the Girls Aloud singer’s diagnosis.
In New! mag, Linda wrote: “Sarah, I was so shocked and devastated to hear about your breast cancer diagnosis.
“If I can say anything to you, the support you’ll get from the public will be amazing – it was overwhelming for my sister Anne [who is also suffering from breast cancer] and me.
“People are genuinely lovely and want to wish you the best. Initially, I didn’t want people to pity me, but it was a weight off my shoulders when it came out and I hope you’re feeling a sense of relief too.”
Harding revealed on social media last week that her breast cancer had spread to other parts of her body.
“I’m currently undergoing weekly chemotherapy sessions and I am fighting as hard as I possibly can,” she wrote on Instagram.
“My amazing mum, family and close friends are helping me through this, and I want to say a thank you to the wonderful NHS doctors and nurses who have been and continue to be heroes.”
In her letter, Nolan wrote to Harding that it was her third time being diagnosed with cancer and that “each time is just as scary.”
“But once you get your head around it – and you’ve got a supportive family, which I know you have – before you know it you’ll be at the end of your chemo treatments.
“I started my chemo in June and thought ‘I’m never going to get to September,’ and now I’ve got two treatments left. And you get into a routine with it.
“Although you’re feeling rubbish, remember it’s treatment to make you better. As you’ve said, the doctors are amazing and give you so much support, but don’t be frightened to ask questions.”
Nolan’s incurable liver cancer comes after a previous fight with breast cancer in 2006, cellulitis and lymphoedema in her arm in 2007 and secondary cancer on her pelvis in 2017.
Nolan told Harding that she “does not have to be brave”.
“But you have to get on with it because the only other option is to be down and depressed,” she said.
“Try to have nice times with special people. Without sounding too morbid, tell those closest to you that you love them and surround yourself with people you really want to be there.
“Your family is so important during this time – to be there for you when you have a meltdown, or when you want to have a laugh.”