The chart-topping band - which includes Cheryl, Nadine Coyle, Sarah Harding, Nicola Roberts and Kimberley Walsh - had been planning to reunite for their 20th anniversary next year, but their ambitions have been put on hold because of Sarah's cancer battle.
Nadine explained to Sink The Pink’s Pop Tarts Instagram Live: "I would love to be like, ‘Yes, this is what we’re doing’. At the minute, it’s very much thinking about Sarah, being there for Sarah.
"It’s a really unfortunate time for her and the whole thing is like, ‘Come on, come on’, because then that could be the priority - talking about, ‘Should we do a tour? Should we do something?’. Instead we’re worried about Sarah.
"We’re just focusing everything there. That is the main priority."
Sarah, 39, revealed she'd been diagnosed with breast cancer last August, shortly after the girls all reconnected ahead of a potential reunion.
The pop star announced the news via her Twitter account.
She wrote on the micro-blogging platform at the time: "Hi everyone, I hope you are all keeping safe and well during these uncertain times. I've not posted on here for so long, thank you to everyone who has reached out to check in on me, it really does mean a lot.
I feel now is the right time to share what's been going on.
"There's no easy way to say this and actually it doesn't even feel real writing this, but here goes.
Earlier this year I was diagnosed with breast cancer and a couple of weeks ago I received the devastating news that the cancer has advanced to other parts of my body.
"I'm currently undergoing weekly chemotherapy sessions and I am fighting as hard as I possibly can.
I understand this might be shocking to read on social media and that really isn't my intention.
"But last week it was mentioned online that I had been seen in hospital, so I feel now is the time to let people know what's going on and this is the best way I can think of to do so. (sic)"