Jon lost out to operatic group Collabro and illusionist Darcy Oake in last week's semi-final, but the judges have now chosen him to go head-to-head with the top 10 acts on Saturday's final.
"I'm on top of the world," said Jon. "I thought I was in with a chance but there were so many strong contenders for the wildcard.
"I thought Bailey had a good chance - he is hugely talented. Also Light Balance, Innova and Jodi."
Jon now has just three days to prepare for the live final, but he was already working on a new routine just in case he got the call.
"I've got quite a few new impressions planned for the final," said Jon. "But I'll be bringing out a few favourites too. I'd say the act will be 75 per cent new material."
Jon is hoping to follow in the footsteps of his impressionist father too. "My dad is absolutely over the moon," he said. "He's so made up for me."
Jon, who has impersonated the likes of Simon Cowell, Ant and Dec and David Beckham, added: "Joe Pasquale tweeted me well done after I impersonated him. I've had loads of comments about my Ant and Dec. That seems to be the favourite."
Jon now completes the line-up for this year's finalists, which are as follows:
1) Operatic group Collabro
2) Illusionist Darcy Oake
3) Essex singer James Smith
4) Dance troupe The Addict Initiative
5) Opera singer Lucy Kay
6) 'Men in Heels' dancers: Yanis Marshall, Arnaud and Mehdi
7) Singing duo Bars and Melody
8) Violinist Lettice Rowbotham
9) Swing group Jack Pack
10) Salsa dancers Paddy & Nico
11) Impressionist Jon Clegg
The winner of BGT will take home £250,000 and bag a spot at the Royal Variety Performance.
Britain's Got Talent live final airs on Saturday 7 June on ITV at 7pm.
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Watch Jon's semi-final performance below: