Future F1 Star Oscar Piastri

Alpine reserve driver Oscar Piastri is calling B.S. on his own team ... saying he will NOT be taking Fernando Alonso's seat next season -- just hours after the organization initially announced the move.

The 21-year-old Australian prodigy went to social media on Tuesday to address Alpine's statement ... saying it's not true and he did not sign off on it.

"I understand that, without my agreement, Alpine F1 have put out a press release late this afternoon that I am driving for them next year," Piastri said on Instagram.

"This is wrong and I have not signed a contract with Alpine for 2023. I will not be driving for Alpine next year."

The denial comes after Alonso's shocking exit from Alpine for…