Louis Tomlinson is mourning the sudden loss of his 18-year-old sister, Felicite Tomlinson, who unexpectedly passed away on Wednesday of a suspected heart attack.

According to The Sun, the One Direction singer’s sister, an aspiring fashion designer, collapsed at her fourth-floor apartment in West London on Wednesday. A person who was with her at the time called 999 (the U.K. version of 911), and emergency responders quickly arrived on the scene.

Despite extensive efforts to resuscitate Tomlinson — nicknamed “Fizzy” — she was pronounced dead at the scene.

RELATED: Louis Tomlinson Puts Pen To Paper For Tribute To His Late Mother In ‘Two Of Us’ Lyric Video

In a statement to Metro.co.uk, the London Ambulance Service said, ‘We were called at 12.51pm. We sent two ambulance crews, a single responder in a car and an advanced paramedic to the scene. Sadly, despite efforts of our medics, a person died.”

Her death is currently being treated as “unexplained” at this stage, but it’s suspected the cause of death was cardiac arrest.

More answers will come after a full post-mortem examination and toxicology tests are carried out.

“Felicite was an absolutely adored young woman who was loved by Louis and her whole family,” a famliy friend told The Sun. “She had so many friends and was such a positive happy person. It’s a massive loss to the world. She was a lovable, caring, bright, passionate, popular and beautiful young lady.”

For the singer, his sister’s death comes just two years after the tragic loss of his mother, Johannah Deakin, who lost her battle with leukemia at age 42 in December 2016.

RELATED: Louis Tomlinson Mourning The Loss Of His Mother, 42, After Leukemia Battle

Tomlinson has reportedly pulled out of a scheduled performance for British charity Comic Relief, with a source telling The Sun, “Louis is obviously distraught at the loss of his sister. He has cancelled all promo, which includes his performance on Comic Relief. It’s just the biggest tragedy and everyone wants to be there for Louis, who is the leader of his family since the loss of his mother and always there for all his siblings.”

Simon Cowell was among those paying tribute to the teenager on social media.

His Instagram account posted:

Click to View Gallery
Stars We’ve Lost In 2019