Sources on set tell TMZ ... extras for the movie have been complaining about working for over 2 hours nonstop and not being allowed to use the bathroom or drink water during filming at Warner Bros Studios in Burbank.

We're told the potty problems are mainly cropping up when extras are on set for camera setups, which often last between 20 minutes to an hour. The background actors fear repercussions if they ask higher-ups to use the loo ... that's because they say some extras have already been berated.

Our sources say one extra had to use the bathroom "terribly" between camera setups, but in order to do so, they first had to "state their case" to the assistant director as to why it was really an emergency.

When the extra was on their way back to set after using the bathroom, we're told they grabbed a cup of water and an assistant director said to them, "Why are you drinking more water? So you have to go pee again?"

We're told another A.D. was overheard saying, "I've never had to go to the bathroom this much. What do they even do in there? Cause they definitely aren't using them."

With the shoot tentatively scheduled for 5 weeks, there's a lot of money and regular work on the table for extras during a normally slow period in Hollywood, which is why some don't want to ruffle any feathers over bathroom breaks.

We've learned SAG received a complaint about a break time violation and they investigated and spoke to production. We're told production's been cooperative and SAG will continue to follow up and monitor the set.

"Joker: Folie à Deux" is set to be released in 2024 ... and it's starring Joaquin Phoenix and Lady Gaga. Filming began in December.

It's the second time there have been bathroom issues on a "Joker" movie set ... as we first told you, during filming for the first movie back in 2018, extras were locked in a Brooklyn subway car for more than 3 hours and started peeing on the tracks.