Astley filed legal docs Thursday in L.A. claiming Gravy's hugely popular "Betty (Get Money)" uses an impersonated version of his own voice from 1987's "Never Gonna Give You Up."

In the docs, obtained by TMZ, Astley says Gravy and his producers -- including Dillon Francis -- "conspired to include a deliberate and nearly indistinguishable imitation of Mr. Astley's voice throughout the song." He's also suing Nick Seeley (aka Popnick), the vocal impersonator.

Astley says he never gave Gravy, or anyone involved in "Betty (Get Money)," permission to use or impersonate his voice -- and because Gravy did so, he's spoiled any future chance for Rick to collaborate with another artist in the future.

We should say, however, Gravy did license the instrumental from Astley's '80s hit ... just not his voice.

What's more ... Astley's lawsuit points to some examples where Gravy allegedly claimed he knew he was in murky territory for impersonating Rick, including an interview with Billboard where he said he "basically remade" Rick's voice, "because it makes it easier legally."

"Never Gonna Give You Up" was a huge hit for Astley, shooting to the #1 spot. "Betty" has been Gravy's most successful song thus far -- reaching Gold in the U.S., Canada and Australia.