The national anthem performance at sporting events is often maligned but Gladys Knight produced a showstopping rendition of the Star Spangled Banner at the Super Bowl.

However, the performance was also engulfed in controversy with betting agencies taking cash on the length of time the anthem would take to sing.

The 74-year-old soul legend brought the house down with a stirring performance some called one of the best Super Bowl anthems ever as plenty on social media raved about Knight's rendition.

But for punters, there was a huge plunge on the time Knight would take to sing the anthem after Las Vegas insider and Fox Sports radio host RJ Bell tweeted the rehearsal times had taken "about 1:59 and two minutes" to complete.

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The length of the anthem has become a popular Super Bowl prop bet but caused a stir on Monday.

The controversy kicked off when Knight finished singing at about 1:49, with many agencies setting the over/under at 1:50. That is, you could bet on the anthem going for longer or shorter than one minute and 50 seconds.

But the soul singer wasn't done at the 1:49 mark, winding up for a repeat of the word "brave" to finish the anthem, which took the length just past the two minute mark.

Sports betting reporter Darren Rovell said it's not the first time this has happened, with Christina Aguilera singing "brave" twice in the 2011 anthem.

The American anthem ends with the line "O'er the land of the free and the home of the brave" and betting agencies are supposed to use the end of the final "brave" as the cut off point for the tune finishing.

However, Knight's repetition of the final word led to widespread confusion. Reports have come through that some agencies are paying out both over and under.

The New York Post had a list of some the best prop bets including coin toss, with the coaching age gap be mentioned through the broadcast, winner of the Puppy Bowl, will any player kneel for the national anthem or will the line "Greatest of All Time" be mentioned in the broadcast to name just a few.