The singer shared a touching Instagram post breaking the sad news to fans, which she also shared through her Twitter page.
"I wish I could have been more like him!"
Along with a photo of her brother as a child, Katy wrote: “This morning, the most beautiful soul I ever knew passed away. He had a cardiac arrest and his heart stopped. I am still in total and utter shock. My one and only brother Andrew Brien did not deserve to die this young. He was the kindest person, he was never spiteful, judgemental or rude.”
She continued: “He had banter for days and I still can't even believe I will never hear that loud laugh or see that Cheshire cat smile again that made whatever worry I had on my mind dissolve.
"When we were little my brother was picked as team captain for everything as he was a bad boy sportsman but would always pick me first for everything to be on his side, even though I was the smallest girl who would never score any goals, he didn't care he just put my interests first!
“He supported me in everything I did, I remember being back stage at one of my first gigs and he was there in floods of tears telling me how proud of me he was! And I was so, so proud of him! I wish I could have been more like him!!
"I have countless memories of him teaching me how to ride a bike, how to swim, dive, build a den, fish and climb and words can not express how much I'm going to miss him!"
Katy, 23, then thanked her family and friends for their support following an accident which left Andrew severely disabled.
“I just want to say a massive thank you to everyone who has supported my family and I these past 18 months, since his accident.
"Andrew had previously had an accident which left him severely brain damaged to the point of being completely paralysed, not being able to talk or move or do anything for him self, almost like a coma state, anyone who knows someone with brain injury will tell you how devastating it is, grieving and missing someone who is still alive.
“I guess he had finally thought fuck this, he is now free of pain and doctors and medication and can now look over all of us, having a good chuckle as he always did…
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“Thank you to his kind, kind friends that have come to visit and have raised money for the causes supporting his condition, we are so, so grateful, I know we are all so thankful to have known him,
“I would give anything to have him back here again, but now we must honour his life, hopefully one day my heart will heal. I love you Brother xxxxxxx."
Our thoughts are with Katy and her family at this difficult and sad time.