According to The Sun, the dog then followed show co-host Dec around the studio.
A source said: "Ant always likes to get up close to the animal acts. He bent down to pat the dog, which promptly went for him.
"But Dec had the opposite effect. The dog wouldn't leave him alone and followed him about backstage."
Ant reportedly confirmed: "My lowlight was in Cardiff. On the same day I was bitten by a dog, chased by a pig and shouted at by a contestant."
The source added: "It was hilarious. The pig came off stage and headed straight for Ant."
Show boss Simon Cowell reportedly suggested that he would like to see a "talented dog" win this year's Britain's Got Talent.
The fourth series of Britain's Got Talent starts on Saturday at 8pm on ITV1.
> First Look: 'Britain's Got Talent 2010'