Niall Horan is celebrating his 19th birthday today, and while he’s tucking into birthday cake in his Irish hometown of Mullingar, his celeb friends have been tweeting birthday wishes.

Niall Horan

First up Harry Styles’ mum Anne Cox tweeted this adorable picture and said: “Here’s another birthday hug for you sweetie @NiallOfficial xxx”

Niall Horan Anne Cox

Olly Murs tweeted: “HAPPY BIRTHDAY @NiallOfficial have a ledge birthday dued!! Wherever you are in the world!! ENJOY!!”

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Harry Styles tweeted: “@NiallOfficial HAPPY BIRTHDAY MATE!! Hope you’re having a good one. Big love .x”

But the mystery comes in the form of a tweet from Alfredo Flores.

He tweeted: “London nights! Chillen wit the bros @justinbieber @zaynmalik and we just left @niallofficial the best bday voicemail! Happy Birthday bro!”

Niall Horan Justin Bieber Zayn Malik

So Justin Bieber and Zayn Malik left Niall a birthday voicemail?!

What could it be?

Maybe a ‘swaggy’ version of the classic ‘Happy Birthday’ song?

Ooh the mystery.

What do you think? Comments please!

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IMAGES: PA & Twitter

What do you think?