The Instagram star and aspiring fashion designer, who had 1.3 million followers on the social media platform, died at her flat in Earls Court, London.
In a statement, Scotland Yard said: “Police were called by London Ambulance Service at 12.52 on Wednesday, March 13 to a residential address... following reports of a female in cardiac arrest.
“A female believed to be aged 18 was pronounced dead at the scene.”
It added: “Police are in the process of informing her next of kin. At this stage the death is being treated as unexplained.
“A post-mortem examination will take place in due course. Enquiries continue.”
According to reports, a person who was with Felicite - known as Fizzy - called emergency services.
Louis was reportedly told of her death on Wednesday.
The former One Direction star has cancelled an appearance on tonight’s Comic Relief.
He was due to perform his new single as one of the headline performers on the BBC1 charity show.
It is the second family tragedy to hit Louis after his mum Johannah died from leukaemia in 2016, at the age of 43.
Last weekend, Felicite was asked if she would be different if her mum had not died, she replied: “Very different, in both good and bad ways, there is collateralized beauty in everything.”
She was also asked about Louis’ new song Two of Us, which he wrote in tribute to their late mum.
She said: “I cried my eyes out in the back of my Uber, but I loved the song filled me with hope.”
Felicite was one of six children. She is survived by Louis, older sister Lottie, twin sisters Daisy and Phoebe and twins Ernest and Doris. She was also an aunt to Louis’ son Freddie.