Sources close to the singer tell TMZ the pair are getting married Thursday in front of a very-exclusive and small guest list of about 100 people. We're told Britney's brother, Bryan, is expected to be in attendance -- but her mom, dad and sister, Jamie Lynn, will not be there.
Our sources say it's not yet been decided who will give Britney away at the altar, as final details are still being worked out.
BS posted a video of herself with Sam Wednesday and her nails were certainly looking wedding ready as she and Sam rode around town sipping champagne in a Rolls-Royce.
Britney and Sam have been living it up during their engagement ... jet setting to Cabo, Cancun, Las Vegas and Hawaii ... where they were riding jet skis, booking lavish suites and dining in the trendiest restaurants.
Unclear what Britney will wear for the big event, but she documented a visit from Donatella Versace in March -- so that could've been a big hint on who designed her dress.
Britney made it clear she wanted to get married, buy a house and have kids following the end of her conservatorship ... so she's close to checking off a significant part of that list.