"I'm over the moon," Dixon told the presenters shortly after. "Lovely place to celebrate it."
The singer beamed as the room erupted into applause, with social media also buzzing with well wishes.
One fan wrote: "Alesha Dixon is expecting?! Congrats @AleshaOfficial! #BGT."
Another said: "OMG ALESHA IS PREGNANT THAT WAS THE BEST INTRO EVER #BGT."
"Congrats to Alesha. What an opening from Ant and Dec #bgt," somebody else posted.
#BGT YOURE TELLING ME ALISHA IS PREGNANT OMG OMG OMG ALESHA CONGRATULATIONS
— ✎Zoe |2/6/19💕 (@PrinceofCrabs)
— Britain's Got Talent (@BGT)
Awwwww Ant and Dec announcing Alesha Dixon’s pregnancy was so cute 😍👶🏼 #bgt
— Bethan-Marie Gibbon.🏴 (@bethangibbonxo)
Alesha Dixon's pregnancy announcement was the absolute best idea ever #BGT
— - rachel truman (@RachelTruman97x)
The former Strictly Come Dancing judge, who is already mum to five-year-old daughter Azura, opened up about wanting to have more children last year but worried she had "left it too late".
"Not a day goes by without me longing for another baby," she told You magazine. "I don’t know whether it will happen given my age but I adore the idea of a large family and I hope I will be lucky.
"The love I feel for Azura is so pure and unconditional. The main thing is to enjoy her in the present and be philosophical about the future, because what will be will be."
Britain's Got Talent continues all week at 7.30pm and 9.30pm on ITV.