Harry Styles and Louis Tomlinson are reportedly barely on speaking terms.
One Direction
The One Direction stars, who performed their last full gig alongside fellow members Niall Horan and Louis Tomlinson in Sheffield last night (30.10.15) before the group takes an extended hiatus later this year, are said to have been embroiled in a bitter feud for years that has been "central to everything."
A source told the UK's The Sun newspaper: "Apart from Harry and Louis, no one knows exactly what caused the falling out -- they didn't even tell the other boys the full details.
"But it was very serious and their friendship has never recovered. Everyone involved in the band is well aware of the animosity between them.
"It's been central to everything else that came next with the band because there was suddenly a massive divide."
The pair have even refused to travel together while on tour.
The insider said: "When the guys started making enough money, Harry insisted there were always two modes of transport, even when it came to private jets.
"The official reason given was that Harry didn't like the fact Louis, Liam and [former member] Zayn [Malik] smoked on board. But it was something much more personal.
"For the making of their last three albums, they never spent any time together in the studio as a band.
"That's quite an extraordinary situation, but their team knew the less time Harry and Louis had to spend together, the better the chances 1D would stay together."
The boy band are set to release their upcoming album, 'Made In the AM,' next month.
Tagged in Harry Styles Niall Horan Liam Payne