Justin Bieber is in mourning.

The popstar is heartbroken after learning one of his biggest fans Avalanna Routh - who had nicknamed herself 'Mrs. Bieber' after meeting her idol in February - tragically passed away yesterday morning (26.09.12)

The 'Boyfriend' singer wrote on his Facebook and twitter pages: ''Just got the worst news ever. One of the greatest spirits I have ever known is gone. Please pray for her family and for her. RIP Avalanna.''

Devastated Justin grew close to his six-year-old fan, who was battling a rare and aggressive form of cancer, after flying to New York City to meet her and her mother for an extra special Valentine's Day date in February.

The 19-year-old star became aware of Avalanna's desire to meet him following a social media campaign called 'Get Avalanna to Meet Justin Bieber' calling on him to grant the sick little girl's biggest wish.

Justin was instantly touched after first meeting Avalanna in February.

He tweeted: ''that was one of the best things i have ever done. she was AWESOME! feeling really inspired now #MrsBieber.''

He initially spent almost two hours playing with the little girl and returned to visit her several times before her death.

Avalanna also became actress Jessica Biel's ''hero'' after meeting they met at a 'Stand Up To Cancer' fundraising event earlier this month.