Liam Payne’s girlfriend, Kate Cassidy, has said she has “lost the best part of myself” after the singer’s death at the age of 31.

A postmortem examination report said former One Direction member Payne died of multiple traumas and “internal and external haemorrhage” after falling from a third-floor balcony of the Casa Sur hotel in the Argentine capital, Buenos Aires.

Police officers and the emergency services found the music star dead at the scene last week and his body was transferred to the judicial morgue.

In a post to Instagram on Wednesday, a week after the singer’s death, Cassidy said: “I don’t even know where to begin. My heart is shattered in ways I can’t put into words.

“I wish you could see the huge impact you’ve had on the world, even as it feels so dark right now.

“You brought so much happiness and positivity to everyone – millions of fans, your family, friends, and especially me.

“You are so incredibly loved.

“You are – because I can’t say were – my best friend, the love of my life, and everyone you touched felt just as special as I did.

“Your energy was contagious, lighting up every room you walked into.

“None of this feels real, and I can’t wrap my head around this new reality of not having you here. I’m struggling to figure out how to live in a world without you by my side.

“Together, we got to be kids again, always finding joy in the smallest things.

“Liam, you had the kindest soul and the most fun-loving spirit. It feels like I’ve lost the best part of myself.

“I can’t imagine a day without your laughter and love. You brought so much light into my life.”

Cassidy said the pair had sat “manifesting our lives together” a few weeks ago and revealed she had a note from the singer, which she has kept “close”.

According to Cassidy, it said: “Me and Kate to marry within a year/engaged & together forever 444”.

“Liam, I know we’ll be together forever, but not in the way we had planned”, she added.

“You’ll always be with me. I’ve gained a guardian angel.

“I will love you for the rest of my life and beyond, carrying our dreams and memories with me everywhere I go.”

Tributes have flooded in for Payne since the news of his death, including from his former partner Cheryl, his One Direction bandmates and music mogul Simon Cowell.

Payne’s bandmates, Niall Horan, Louis Tomlinson, Zayn Malik and Harry Styles, said in a joint statement that they were “completely devastated” and will miss the singer “terribly”, adding the “memories we shared with him will be treasured forever”.

The singer’s family said they were “heartbroken” after his death, adding: “Liam will forever live in our hearts and we’ll remember him for his kind, funny and brave soul.”

Payne found fame alongside his One Direction band members when The X Factor creator Cowell put them together to form the boyband on the ITV talent show in 2010.

Payne first auditioned for The X Factor in 2008 when he was 14, singing Frank Sinatra’s Fly Me To The Moon, with judge Cowell telling him to return to the talent show two years later.

Payne previously said he struggled with alcoholism at the peak of his success with One Direction, describing hitting “rock bottom” to The Diary Of A CEO podcast host, Steven Bartlett.