Liam Gallagher SUES Noel: Apologize for Calling Me a Drunk

Liam Gallagher claims he's suing his brother Noel for telling the world Oasis bailed on a 2009 music festival because Liam was too "hung over to perform" ... insisting he can PROVE Noel was lying.

Liam just told U.K. tabloid The Sun: "I have taken legal action against Noel Gallagher for statements he made claiming Oasis pulled out of the 2009 V festival Chelmsford gig because I had a hangover.”

Liam claims the truth is he was suffering from laryngitis -- and he has a doctor's note to prove it.

Liam says he's pissed because his bro's comments went "way beyond rock'n'roll banter and questioned my professionalism."

He adds, "I tried to resolve this amicably but have been left with no choice but legal action. All I want is an apology."