Before comedian Ned Woodman took to the stage, the pair had a small falling out when Ant McPartlin made a swipe at his co-presenter's size.
Introducing the youngster, Dec informed viewers at home: "Next up is the nine-year-old stand-up who has taken the mickey out of the judges twice.
"And, I'll tell you what, after this show finishes, I would not like to be in his shoes."
Taking the opportunity to make a joke at his Geordie co-host, Ant remarked: "They'd fit you though."
Clocking on to his insult, Dec snapped: "What?!"
Britain's Got Talent: Ant and Dec were introducing Ned when the remark was made
Britain's Got Talent: Dec appeared less than impressed with Ant when he joked about his shoe size
"His shoes. They'd fit you. He's a size three as well," Ant quipped.
Unimpressed by Ant's joke, he hit back: "Don't be ridiculous! I'm a three and a half."
Of course the pair were simply joking with one another and the segment was all in jest as Dec later went on to mock the size of Ant's forehead.
Claiming he could read minds like tonight's act DNA, he noted that he would have to place his hands on his head.
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Britain's Got Talent: Dec snapped at Ant when he compared him to a child Britain's Got Talent: Most memorable moments Mon, May 1, 2017
Britain's Got Talent: The best and worst moments from the ITV talent show.
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Dance act Just Us get the golden buzzer
Britain's Got Talent: Dec later joked about the size of Ant's forehead
As he laid his hands on Ant's forehead, he laughed: "Oh, I might need two hands for this one."
Tonight's final saw the likes of the Missing People Choir, MerseyGirls, DNA and Sarah Ikumu battle it out to win a place at the Royal Variety Show.
Mind-reading duo DNA stumbled in their performance when they made a minor mistake during their performance, which left the judges scratching their heads.
Tokio Myers was crowned champion of this year's competition and collapsed on the floor when the announcement was made.