Simon Stephenson -- whose story/writing credits include 'Luca,' 'Paddington 2' and other big hits -- levied a heavy plagiarism allegation against Alexander Payne earlier this year with none other than the WGA ... and his claims are just now coming to light publicly.

According to Variety -- which has reviewed and published emails from this whole ordeal -- Stephenson hit up some WGA honchos to complain that Payne's script/story for 'The Holdovers' was a "brazen" ripoff of a script he wrote back in 2013 called 'Frisco.'

Not only are the plots nearly identical, Stephenson alleges, but he goes on to claim there is "line-by-line" replication ... adding he has proof Payne reviewed his screenplay in years past.

Stephenson wrote in one of his emails to WGA, "I’ve been a working writer for 20 years -- in my native U.K. before I came to the U.S. -- and so I’m very aware that people can often have surprisingly similar ideas and sometimes a few elements can be ‘borrowed,’ etc."

He adds emphatically ... "This just isn’t that situation. The two screenplays are forensically identical and riddled with unique smoking guns throughout."

Stephenson was reportedly told his gripes were not a guild issue, and that he might need to consider a lawsuit to address his concerns -- no word on whether he's done that yet.

As for Payne -- who's not a writer on 'Holdovers,' but who says he heavily supervised the script-writing process -- he's publicly said his flick is loosely inspired by a 1935 French film.