A J-Pop star has died at just eight years old – only five days after performing on stage.

Hirata Runa, known by her stage name Luna-chan, died at around 4am on Friday, September 25 after suffering inflammation of the heart muscle linked to the flu, according to her agency.

Hirata had posted a cheerful message thanking her supporters following her performance.  

The star was a year three primary school pupil and a trainee for the DIANNA Project, a Nagoya-based talent programme that develops and markets young J-Pop 'idols' based on their personality and public image. 

The eight-year-old had performed at the project's Regular Live Vol.70 concert on September 20.

Following the performance, she wrote in her final public post: 'Thank you again today. It was so much fun. I hope the people in part three have fun too.' Hirata died just five days later.

Her agency said: 'Nagoya trainee Hirata Runa has passed away due to myocarditis resulting from influenza.'

No further medical details or treatment history have been released.

The agency said Hirata's wake and funeral had already taken place with only close relatives attending.

It added: 'We ask that people do not make private inquiries, seek interviews or raise questions about this on social media.'

Hirata's devastated family also shared a message through her official account.

They wrote: 'A third of Runa's life was as an idol. Runa loved everyone. If possible, please let her stay in a corner of your hearts as an eternal idol.'

They also thanked 'friends who were kind to her, fans who supported her, the office that made her an idol, and everyone who was part of her life'.

Fellow trainee Miyata Ryoka, a member of the Japanese idol group Kocchi Muite Darling, said the news had left her 'at a loss for words' and that she would pause her social media activity to mourn privately.