EXCLUSIVE

Lil Wayne is ready to take his feud with Birdman to the next level ... threatening to drag him and Cash Money into court if they don't drop his album NOW.

Sources very close to the feud tell TMZ ... Wayne has issued a simple ultimatum, in writing, no less -- release "Tha Carter V" pronto ... or he will sue to leave Cash Money altogether.

We're told Wayne is standing firm in his belief the album is ready to go -- but Birdman still says there's a lot of behind-the-scenes work left to be done.