Sorry, One Direction fans ... a reunion seems ever less likely to happen -- because a new report claims Zayn Malik slugged Louis Tomlinson.

The Sun laid out the alleged altercation in an article posted late Friday ... claiming the fight went down during the filming of a new Netflix reality show which was supposed to focus on the pair bonding on a road trip.

The outlet says sources told them the fight was triggered when Malik made a crack about Tomlinson's mom ... who died of cancer in 2016 -- not the kind of quip friendships usually come back from.

Tomlinson allegedly took a step toward Malik -- who sources told The Sun then punched him right in the face.

Malik was allegedly wearing rings ... and, he cut his former bandmate's head -- drawing blood and concussing Tomlinson.

The incident took place about 6 months ago ... and, The Sun claims the two haven't spoken since.

According to reports ... Tomlinson has unfollowed Malik -- the ultimate end to strained relationships in the modern age.

We've reached out to Malik and Tomlinson's teams ... so far, no word back.