Melissa Joan Hart feels a lot of regret over introducing a then-underage Britney Spears to her first clubbing experience in the nineties ... something she says she wishes she didn't do.

BS and MJH's friendship began in 1999 when the 'Sabrina' actress appeared in Spears' "(You Drive Me) Crazy" music video, and Britney guested on Melissa's ABC show.

Being 6 years older, Melissa says she felt a sibling-like bond with Britney ... hence why she feels she should've known better than to take her to a nightclub at 17.

Melissa gave some more context to Entertainment Tonight ... saying she extended the nightclub invite to Britney after noticing she could never break free from the swarm of people constantly surrounding her.

She didn't think it'd be a big deal ... adding, "I would go to a club every night -- I love dancing, and I loved going out, but I also knew to be responsible and, like, when to stop."

Recalling their first night out together, MJH added ... "She was underage and young and -- but I [was] just like, 'Let's go out. We're just gonna go out and have some fun.'"

In the years that followed, though, Britney became known for hitting the party scene way more often, especially with newfound gal pals like Paris Hilton and Lindsay Lohan in the early aughts. At the time, they were like a little crew hitting the town almost every weekend.

Now, Melissa says she thinks she may have unintentionally opened the floodgates to Britney's wild adventures, with her explaining ... "I feel really guilty about that still to this day because I should have known better, being a big sister."

Of course, while Britney continued to enjoy career success, things quickly spiraled, and she was placed under a conservatorship by her father, Jamie -- which ended after 13 years in 2021, thanks in part to the #FreeBritney movement.

However, there've been calls to reinstate a conservatorship for Britney after her life seems to be unraveling of late ... especially following her recent erratic behavior with her current BF.