Sarah Harding agreed to a 20th anniversary Girls Aloud reunion just before her breast cancer diagnosis.
The 38-year-old singer - who recently revealed she has been secretly battling advanced-stage breast cancer - held discussions with bandmates Kimberley Walsh, Nadine Coyle, Nicola Roberts and Cheryl Tweedy about reuniting to celebrate the group's milestone anniversary.
A source told The Sun on Sunday newspaper: "The gang started early discussions at the beginning of last year.
"It was all very cloak and dagger, but the girls were all hopeful of agreeing contracts later this year and getting concrete plans in place."
However, it is believed the plans have been put on hold, while Sarah's bandmates rally round her as she is treated for the disease.
The insider added: "Poor Sarah was desperate to make this happen, so her diagnosis has been doubly devastating.
"At the moment, all anyone cares about is that she gets the treatment she needs -- the music comes second.
"But for the first time in years, all five girls are back in regular contact -- even if under the most heartbreaking of circumstances."
Sarah recently revealed her diagnosis via social media, explaining she was diagnosed earlier this year and it has now spread to other parts of her body.
She wrote: "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)"