Tomlinson, who was a social media star in her own right with almost 1.3 million Instagram followers, was reportedly found unconscious at her apartment in Earls Court, West London on Wednesday, according to The Sun. Félicité had a large following of her own on Instagram, where she occasionally shared photos of herself with Louis and their other siblings.
His mother Joannah died in 2016 after a battle with leukemia.
How did Louis Tomlinson's sister Felicite Tomlinson die? The "Attention" singer sent his love and prayers.
In a statement to Metro.co.uk, the London Ambulance Service said, 'We were called at 12.51pm.
Despite extensive efforts to resuscitate Tomlinson - nicknamed "Fizzy" - she was pronounced dead at the scene. A person who was with her at the time called 999 (the United Kingdom version of 911), and emergency responders quickly arrived on the scene.
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Her last social media post came on Sunday (March 10).
Felicite was well-known on social media, with 1.3 million followers on Instagram.
It is understood that Tomlinson, who is reportedly in London for a now-cancelled appearance on Friday night's Comic Relief, was told of her death. Currently, The Sun says the young woman's death is being classified as "unexplained" by authorities, pending a full autopsy and toxicology test.
He previously released song Just Hold On - his debut solo single - in her honour of his mum and performed it on The X Factor just days later.
American singer-songwriter Charlie Puth said he is sending "prayers" to Tomlinson and added: "I can't imagine how hard it is right now".