The cast and crew of How I Met Your Mother have been forced to wear wristbands on the U.s. sitcom's set while Britney Spears is filming her cameo role - to prevent passers-by from snapping the singer at the studio.

The show's star Neil Patrick Harris was mistakenly refused entry to the stage - because he wasn't wearing one of the exclusive bands security officials issued to keep civilians out.

Harris tells Eonline.com, "It's been great. We're required to wear wristbands all day. It keeps meddling blogger friends from just strolling on to set to snap camera-phone pics. They have different wristbands every day, and it's like the ultimate after-party.

"I wasn't given a wristband, because I'm on the show, but then they swapped out security people and they wouldn't let me on the stage because I didn't have the proper wristband. We had words. I did eventually make it on set, because I just kept going through."

"But there were lots of 'Hey, hey - I need to see your wristband,' and I said, 'Lady, I'm actually on the show, so I don't have a wristband.' That's the kind of excitement there is on Stage 22 right now, mid-Britney."

The show featuring the troubled star is set to be aired on March 24th on U.s. cable channel Cbs.