Britney Spears Dines In No Alcohol Zone -- BUT ...

Britney Spears dined out last night with her boyfriend and a bunch of his friends ... and alcohol became an issue.

The group of 8 went to dinner at the Four Seasons Hotel in Westlake Village, near L.A. We're told before Britney arrived, a few people in the group ordered drinks at the bar. They all then went to their table in the restaurant, leaving the drinks behind.

Here's the deal ... Britney -- who once had a serious drinking issue that caused the custody judge to order her to undergo random testing -- has laid down the law with friends -- no one drinks when she's at the table. Someone who was there tells TMZ ... a waitress came by and brought a glass of wine that one of the women had been drinking, and apparently the woman knew the drill and said the waitress was mistaken -- she wasn't drinking wine.

Apparently ... it was too much for at least one of the people to take. We're told once Britney left ... one of the guys at the table made a beeline for the bar, ordered 3 vodkas in rapid succession and got kinda wasted.

Cheers.