The 21-year-old told her Instagram followers of her diagnosis with a post on her Stories on Thursday.
Demi, who appeared on last January’s series of the ITV2 reality show, said: “I got my results today and unfortunately I have thyroid cancer. The tumour has been removed but I’m now due to have more surgery to remove the rest of my thyroid.”
Demi said she is “staying very positive”, adding: “I’m a strong girl so I’ll be fine.
“Thank you for your love and support always. I’ll bounce back stronger.”
The reality star has been keeping her followers updated about her health after first discovering a lump in her neck in early April.
She was admitted to hospital where she had the “potentially cancerous” lump removed and has been awaiting the results of further tests.
Thyroid cancer is a rare type of cancer that affects the thyroid gland, a small gland at the base of the neck that produces hormones.
It is most common among people in their 30s, while women are two to three times more likely to develop it than men, according to the NHS.
Demi rose to fame when she entered the Love Island villa in South Africa during the first winter series in January 2020.
She made it to the final with partner Luke Mabbott, finishing in third place, although the couple later split.
Shaughna Phillips, who appeared on the 2020 series alongside Demi, sent love and support to her friend on Instagram following her diagnosis.
Posting a video of them in the villa together, she wrote: “My sweet, gentle, beautiful girl, the same message I was giving you in the video applies today more than ever. Game face on my girl, you’ve got this.
“I love you soooo much! And I am SO lucky to have you in my life.”
If you have any questions or concerns about cancer, the Macmillan Support Line is open every day from 8am-8pm on 0808 808 00 00. Further information and support is available online at macmillan.org.uk.