Showbiz
Rex Features
The 'Just Dance' star was putting on an outfit backstage at the Brit Awards when Tennant burst into her dressing room.
According to The Sun, he quickly shut the door and shouted: "I haven't seen a pair of those in a while."
The singers, who teamed up with Killers frontman Brandon Flowers for a medley of hits, are reported to have laughed about the incident for the rest of the evening.
"Lady GaGa thought it was hilarious. They were both in hysterics - it was their joke before they went on stage," a source told The Mirror.
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