KFED BOMBS IN BIG APPLE -- EVEN BRIT BAILS ON HIM!

Kevin Federline's NYC concert on Nov. 4 at Webster Hall only attracted 300 fans — the venue can hold 1,500! — and many of the concert-goers didn't even stay for the whole concert! And the biggest shocker of the night? His wife Britney Spears was a no-show!

But Kevin should be grateful the concert ever happened. A bartender told Star, "They were going to cancel this concert, but he begged them to keep it on! He had to fight with them to keep this concert."

Kevin was supposed to go on at 7 p.m., but, said the bartender, "he was holding things up because there weren't enough fans! He was backstage waiting for more people to show up. It was a bad idea — he shouldn't have neglected the fans that were there."

Kevin finally went on at about 10 p.m. — three hours late. "He had few prepared dance moves, and seemed really confused about how to act on stage. He would pace from one side of the stage to the other, just saying things like 'Hey' 30 times in a row, or 'New York, thank you for coming — buy my CD.'"

Kevin only rapped for a half hour, and by the last song, "only one-third of the audience had stayed until the end!" an eyewitness told Star.

An audience member told Star, "If I had a bucket, I would vomit in it right now."

Kevin also had an album release party at NYC's Stereo nightclub on Nov. 2, where Britney again was a no-show. When asked where his wife was, he said, laughing, "I don't know...at home with the kids."

Stay tuned for more inside scoop on these two....

— MIRELLE ARGAMAN