The clock's tickin' on Tyson Fury's reign as WBO heavyweight champ ... 'cause the boxing org. hit the fighter with an ultimatum ... tell us why you should keep your belt, or lose it.
In case you missed it ... Fury pulled out of his Oct. 29th rematch with Wladimir Klitschko after testing positive for cocaine.
Now, the WBO has spoken ... telling Fury he has 10 days to "show cause why your title should not be vacated due to inactivity, breach of contract and performance enhancing drugs and stimulants."
The letter was delivered Oct. 7th ... giving Fury til' October 17th to respond.
And if he doesn't answer? A rep for the WBO tells us he'll likely be stripped of his title.
Regardless, the rep says they'll decide Fury's fate next week at the WBO convention in San Juan.
Clock's tickin', Tyson -- and ya got some 'splainin' to do.