Four-year-old who went viral after her father shared a photo of her cancer struggle has died.

A four-year-old girl who became synonymous with childhood cancer when her father began posting about her health battle online has died.

Jessica Whelan died on Sunday from neuroblastoma, an aggressive cancer of the nervous system mostly affecting babies and children.

Jessica had an aggressive cancer of the nervous system. Source: Facebook

In a moving post on her public Facebook page Jessica's dad, Andy Whelan, said goodbye to his little girl and broke the sad news to the world.

"I feel both sadness and relief in informing you all that Jessica finally found peace at seven o’clock this morning," he wrote.

"No longer does she suffer, no longer does she feel the pain of the physical constraints of her body."

Andy said his princess has grown her angel wings and was not watching over her family, including her younger brother, from heaven.

"Last night she finally allowed me to hold her in my arms and we had a big cuddle as I told her how much I loved her," he wrote.

"I told her again that it was okay for her to close her eyes and go to sleep and I kissed her forehead and her lips numerous times.

"It seems like this is what she needed to finally allow her to find comfort in her passing as within eight hours of this cuddle she finally took her final breath."

Jessica's father said goodbye to his "princess"  in a heartfelt message. Source: Facebook

Jessica, from the small English village Oswaldtwistle, was diagnosed in October 2015 and after a year of treatment her cancer had progressed beyond the point of return.

Just last month, her oncologist told the Whelan family her expected life would now be measured in months not years.

The family set up a GoFundMe page in Jessica's honour, which has now raised the equivalent of more than $160,000.

Andy said his daughter remained "daddy's girl" right up until her final moments.

"I feel like a massive part of me has just been torn away but I am so glad that I could give her that comfort in her final hours. She passed peacefully and calmly with not even a murmur."

He thanked everyone for their support and asked that the public respect the family's privacy as they mourn.

Feature image: Facebook