The next full moon isn't slated to occur until March 14 ... but John Fury couldn't wait a few days -- flashing his bare rear to the cameras while out for a stroll across the pond on Monday!!

John, Tyson, Tommy and Shane Fury were spotted leaving a café in the UK ... with the 59-year-old deciding the best pose for the cameras was to drop trou'.

The former world heavyweight champion didn't seem to be amused by his father's actions -- opting to keep it casual with the cameras.

Tommy split from the rest of the group to hitch a ride in his G-Wagon ... with his muscles clearly putting in some work on his shirt.

Whether it was just a visit to the café or a part of a more busy day, it marked some quality family time for the Furys regardless. Tyson recently announced his retirement in January ... following back-to-back losses to Oleksandr Usyk.

Tommy was supposed to fight former UFC middleweight contender Darren Till back in January ... but withdrew from the bout -- claiming Till would not "abide by professional boxing rules."

As for John ... he's continuing to prove he has nothing to hide.