EXCLUSIVE

Stan Lee's ex-business partners took advantage of a grieving old man battling to save his sight and essentially stole his good name ... according to a massive new lawsuit.

The comic book legend is going after POW! Entertainment -- including current CEO Shane Duffy, and Gill Champion, with whom he co-founded the production company -- for a whopping BILLION BUCKS ... claiming they either tricked him into signing over the exclusive rights to his name, image and likeness -- or flat-out forged his signature.

Here's the backstory -- POW! was sold to a Chinese company called Camsing in 2017, but during negotiations, Lee claims he was in poor health and dealing with vision loss due to macular degeneration. He was also grieving over the death of his wife, Joan.

According to the docs ... Lee had no idea a sale was in the offing. Apparently POW! had to get a sign-off from Stan to use his name, and he claims he would NEVER have put his John Hancock to paper. The lawsuit calls Stan's name "the most important and prized possession Lee and his family owned his entire life."

Lee also points to the fact he even refused to give Disney the right to use his name when he struck a multi-billion dollar deal with the movie company.