Actor and talkshow host James Corden said the news was "incredibly sad" and tweeted to Tomlinson that he is "not on your own in this".
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.
Singer Charlie Jones tweeted that he "can't imagine the pain" Tomlinson and his family are going through.
A spokesperson for the Met said: "Police were called by the London Ambulance Service at 12.52pm to a residential address in SW5".More news: Mercer Global Survey: US Ranks Low in Quality of Living
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Her death is now being treated as "unexplained".
Efforts were made to resuscitate the young girl but she was pronounced dead at the scene.
A person who was with the teenager called 999, according to The Sun newspaper. "A post-mortem examination will take place in due course".
"We sent two ambulance crews, a single responder in a auto and an advanced paramedic to the scene".