Blake Lively is ready for a major development in her ongoing legal war with Justin Baldoni ... 'cause she'll be sitting for a deposition soon ... TMZ has confirmed.

A source with direct knowledge tells TMZ ... Baldoni's legal team will depose Blake next Thursday in New York. The exact location, we're told, has not been determined.

Our source tells us it'll just be Blake and her legal team in the room ... husband Ryan Reynolds isn't expected to be in the deposition proceedings.

We're told Blake is ready -- she's been ready for a while and insists her story has never changed.

This marks one of the bigger moments in the ongoing legal drama between Lively and Baldoni ... since his $400 million countersuit was tossed last month.

Justin's side originally claimed they were going to amend the dismissed lawsuit ... but then changed course and said "our clients will be pursuing additional legal options that are available to us."

The deposition update marks 7 months since Lively filed her lawsuit against Baldoni and his company Wayfarer Studios over allegations of sexual harassment during the production of their film "It Ends With Us" ... as well as an alleged smear campaign against the actress.

We've reached out to reps for Lively and Baldoni ... so far, no word back.