Polly Noble was diagnosed with cervical cancer at the age of 24, but said she wanted her illness to inspire others, and make them want to live a healthier life.
Katherine Jenkins posted a photo of herself and Polly with the caption: 'Heartbroken. Rest in Peace my darling friend.'
The girls met when they were both struggling singers in 2001, with Katherine once tipping Polly as the 'true star' out of the two of them.
But in 2005, Polly was diagnosed with cervical cancer, and had to share the terrible news with her friend - who then supported her as she went through surgery, chemotherapy and radiotherapy.
'She was too young; she had too much to live for' Katherine said.
Although treatment originally worked for Polly, allowing her to move back to London, she then found a lump in her neck in 2010 that turned out to be a secondary tumour.
'This time her oncologist told her that although she could have more treatment, the cancer was ultimately incurable' said Katherine, adding: 'Neither of us cried. We were too stunned.'
The star shared two pictures of the pair on Monday afternoon, with many followers lending their support to the singer.
One wrote: 'Praying for you Kath as you process her loss and celebrate her life. She's left a huge legacy.'