Her death is being treated as unexplained by police.
Louis was reportedly set to perform "Two of Us", his latest single dedicated to his mom, Johannah Deakin, who died of leukemia in 2016. She lived in the Earls Court area of West London, England.
Celebrities - such as James Corden - sent their wishes to Louis Tomlinson following his sister's death.
49ers, Chiefs agree to terms on Dee Ford deal
Earlier this month, the Chiefs applied the franchise tag to Ford, but they have been open to trading him for the right price. ESPN's Josina Anderson also reported Ford would receive a five-year deal for $87.5 million once the trade becomes official.
It read: "Absolutely heartbreaking hearing the news of Louis sister Felicite sending my love to him & all his family right now! Love to you brother I'm so sorry and my prayers are with you".
Dancing On Ice star Brian McFadden continued: "This is so sad. You're not on your own in this", before assuring Louis that people are pulling for the Tomlinson family.
Felicite Tomlinson has tragically died aged 18.
A spokesperson for the Met said: "Police were called by the London Ambulance Service at 12.52pm to a residential address in SW5".
Minnesota Vikings sign Anthony Barr: Grade, reaction and more
Given that Barr will now be doing his thing as a pure pass rusher in the Jets' 3-4 scheme, we're expecting big things from him. For his career, he has 13.5 sacks while averaging 68 tackles per season over five seasons.
A person who was with the teenager called 999, according toThe Sun newspaper. "A post-mortem examination will take place in due course".
Her last social media post came on Sunday (March 10).
The aspiring fashion designer and social media influencer is thought to have suffered a heart attack, though this is yet to be confirmed.
Trump Budget Sees $1 Trillion Deficits for Next Four Years
Tax cuts have been a priority for the Republican White House and Congress in recent years, rather than deficit reduction. Trump's budget also calls for the extension of the 2017 tax cuts, which are now expected to expire in 2025.