Linda Nolan has said people who said false or mocking things about the Princess of Wales before her cancer treatment was revealed should apologise.
The singer, 65, part of the family pop group The Nolans, is also being treated for cancer.
Apologies have come from figures including the actress Blake Lively and columnist Sarah Vine following Kate’s cancer diagnosis announcement.
Nolan told ITV’s Good Morning Britain: “Of course they should say sorry.
“But, you know, sorry is an easy word to say sometimes and they may be doing it because other people are doing it.
“They should never have even thought it in the first place, to put down lies and horrible words. Sorry doesn’t cut it sometimes.”
In a video message released on Friday, Kate said she was in the early stages of preventative chemotherapy following her January visit to hospital for abdominal surgery.
Discussing the announcement and whether the senior royal should have disclosed more, Nolan said: “I think she did right. I think we all wondered what type of cancer it was. I think she did well, in the fact that she came out and told us when she didn’t have to.
“Who says they have to tell us their life story? They are doing their jobs.
“If she wants to keep it secret and her treatment secret, then that’s how she’s going to handle it and good luck to her.”
Nolan was initially diagnosed with breast cancer in 2005 and the disease resurfaced in 2017 and has since spread to her brain, bones and liver.
Addressing Kate’s message in her video that people struggling with cancer are not alone, Nolan said: “I thought it was perfect. I’m suffering myself and I listened, and I took everything in, and she’s so right for someone who has been diagnosed.
“Hope – you’ve got to have hope and people around you that love you.”
Hollywood actress Lively seemingly mocked Kate after she issued a personal apology for manipulating a family photograph, sparking conspiracy theories about her whereabouts.
Referencing a now-deleted Instagram post, Lively said: “I’m sure no-one cares today but I feel like I have to acknowledge this.
“I made a silly post around the ‘photoshop fails’ frenzy, and oh man, that post has me mortified today. I’m sorry. Sending love and well wishes to all, always.”
Mail on Sunday columnist Vine wrote: “We all, to a greater or lesser extent, owe the Prince and Princess of Wales an apology – myself included – for giving them such a hard time over that doctored Mother’s Day photo, as well as Prince William’s last-minute decision to pull out of his godfather’s memorial service.”