The star, who appeared on the seventh series of Big Brother in 2006, had been fighting anorexia.
Former Big Brother star Nikki Grahame has died at the age of 38.
The TV personality had checked into a private hospital last month as she fought an eating disorder.
In a statement to the BBC, Grahame’s manager confirmed “with immeasurable sadness” that Grahame died on Friday morning.
“Please respect the privacy of Nikki’s friends and family at this tragic and difficult time,” he added.
Former Big Brother presenter Davina McCall led the tributes, tweeting: “I am so desperately sad to hear about Nikki Grahame. My thoughts are with her friends and family xx she really was the funniest, most bubbly sweetest girl.”
Grahame, originally from Northwood in north-west London, took part in the seventh series of Big Brother in 2006.
She became known for her tantrums, particularly in the Diary Room.
Grahame would often complain to Big Brother about the conditions in the house or fellow contestants, famously shouting “It’s so cold,” and, on another occasion, asking “Who is she?” after a new housemate joined the show.
She was hugely popular with viewers, and was voted back into the house four weeks after she was first evicted. She subsequently finished in fifth place.
Speaking to The Guardian last year, Grahame said: “I hold fond memories of Big Brother, even though there were times where it looked like I was having a meltdown.
“Those meltdowns really were me, though. What you see has always been what you get – I can’t be any other way.”
After her appearance on Big Brother, she won a National Television Award for most popular TV contender.
Grahame returned to the house in 2010 to take part in Ultimate Big Brother, the final series of the show to be broadcast on Channel 4.
The show saw several popular Big Brother contestants from over the years return to take part in one final series, before it moved to Channel 5. Grahame finished in second place.
By this time, she had developed her career as a popular TV personality, and starred in her own reality TV series called Princess Nikki.
She also guest-starred on programmes such as the Friday Night Project and as a contestant on celebrity editions of shows such as Pointless and The Weakest Link.
Prior to finding fame on Big Brother, Grahame was an aspiring actress. She appeared as an extra in the BBC’s EastEnders and played a footballer’s wife in Sky One’s Dream Team.
Grahame also appeared as a contestant on ITV’s Blind Date and competed in the 2004 Miss Hertfordshire pageant.
She continued her relationship with Big Brother, returning for 2015’s “Time Warp” twist on Channel 5, and made a cameo in the final series of Big Brother in 2018.
She also appeared as a “wildcard” housemate in the 2016 series of Big Brother Canada.
Grahame’s battle with anorexia was well documented, and she released a book about her eating disorder in 2009.
Last month, a fundraising page was set up, asking fans to donate towards her recovery. It eventually raised more than £65,000.
The page said the 38-year-old was in “a very bad way” and needed treatment in a rehabilitation centre. Several Big Brother figures including former winner Rylan Clark-Neal retweeted the appeal for donations.
On Saturday, the page was updated to announce the news of Grahame’s death.
“It breaks our hearts to know that someone who is so precious was taken from us at such a young age,” it said. “Nikki not only touched the lives of millions of people, but also her friends and family who will miss her immensely.”
Paying tribute to Grahame on Twitter, Clark-Neal wrote: “Thinking of [Grahame’s mother] Susan and Nikki’s close friends and family. A Big Brother Icon.”
TV presenter Paddy McGuinness added: “Definitely one of the stars from the glory years of reality TV. I met her a long time ago and she was a lovely girl. RIP young lady.”
Channel 4 tweeted: We are desperately saddened by the tragic news about Nikki Grahame and our thoughts and deepest sympathies are with Nikki’s family and friends.”
Big Brother Canada added:“Our hearts break for the loss of Nikki Grahame. Nikki will forever be a beloved part of the Big Brother family. Rest in peace.”