Fans have heard the real Britney Spears, perhaps for the first time, and are losing their minds.

An Autotune-free version of the singer's 2004 single, Toxic, has leaked online and the reaction has been surprisingly positive. The raw version of the hit single was posted to YouTube on 5 June and features Spears, 35, singing without alterations to her vocals.

Reacting to the original version, one fan tweeted: "Have you heard Britney Spears' "toxic" without autotune? It's surprisingly good," while another agreed: "Britney Spears singing Toxic wo autotune is amazing." One fan enthused: "I will be listening to the leaked non-autotuned version of Toxic from now until eternity," while another chimed in: "Britney Spears singing Toxic without autotune has changed my life."

Toxic, released in 2004, appears on Spears' fourth studio album In The Zone and won the pop star a best dance recording Grammy in 2005. The rare leak comes just days after Spears was caught lip-syncing during a tour show in Tokyo, Japan on 4 June.

While performing I'm A Slave 4 U, video footage shows the singer pull the microphone away from her mouth while trying to free it from her tangled hair – but her vocals are still heard.

Spears has been plagued by lip-syncing claims throughout much of her career. In 2016, the Baby One More Time singer was said to have mimed at the MTV VMAs while performing Make Me and she came under fire for lip-syncing during her Carpool Karaoke segment with James Corden.

In 2014, Spears' manager was forced to admit that the pop star does not always sing live while defending performances on her Piece Of Me residency in Las Vegas. Adam Leber told Medium.com at the time: "To put on the show that she puts on, it's virtually impossible to sing the entire time and do what she does. She's singing on every song, basically, when she has the ability to sing. There's no way you can dance for 90 minutes straight and sing the entire time."

Spears will resume her Vegas residency at The Axis on 9 August before bringing the four-year stint to a close in December.

Listen to Britney Spears' raw vocals on Toxic:

Listen to Britney Spears' official Toxic single: