The five-piece group sang a rendition of Bring Him Home from the musical Les Miserables, earning a standing ovation from all four BGT judges.
"You are absolute stars," David Walliams told them. "Your voices are perfect together."
Simon Cowell added: "The thing I really like about you is you have decided what kind of band you want to be. The harmonies in that performance were spot on. Britain really has got talent."
Speaking to hosts Ant and Dec following the results of the public vote, the boys said they were "ecstatic" to be put through to next week's BGT final and thanked their "Collaborator" fans for their support.
Collabro will be joined in the final by Canadian illusionist Darcy Oake who won his place after being pitted against impressionist Jon Clegg for the final spot.
The judges had difficulty deciding on which act to choose, with David and Amanda Holden voting for Jon and Alesha Dixon and Simon deciding on Darcy.
Due to the split vote, the decision was brought back to the public and Darcy was awarded a place in the final.
He performed an illusion which involved disappearing from a covered chair and appearing on stage as a member of the Britain's Got Talent crew.
That was something very different.
Simon Cowell to Darcy Oake
"I cannot wait to see what you do next," Alesha told him.
Amanda added: "You are such a professional and you have a real commanding presence on the stage."
Despite missing out on the final, Jon Clegg won high praise from the judges for his comedy routine, which included a number of celebrity impersonations such as Michael McIntyre, Paul O'Grady and Alan Hansen.
But it was Jon's take on BGT host Dec that got the biggest laugh, though Dec himself did not seem as impressed.
"That routine was effortless," said Amanda. "You were on the money with everyone."
Simon added: "That was very clever, you hand the audience in the palm of your hand."
It was a disappointing night for David's golden buzzer act Christian Spridon, who performed a high energy rendition of It's Raining Men.
Eurovision was two weeks ago.
Amanda Holden to Christian Spridon
While David championed his act, the rest of the judges were not as impressed, with Simon describing his performance as "by far the worst vocal ever on Britain's Got Talent".
Alesha told him: "It was so out of tune, it was ridiculous!"
In reply, David told Christian not to listen to Alesha who had "made a career out of miming".
This prompted the former Mis-Teeq singer to empty a glass of water over his head, earning a high five from Simon.
Line-dancing group CountyVive opened the first semi-final, but while David, Amanda and Alesha all praised the group, Simon felt the routine was "a bit of a mess".
The 18-strong act is made up of line dancers from all over the UK, including Fred Whitehouse and Shane McKeever, from Co Tyrone and Laura Carvill, from Co Armagh.
Other acts performing on Monday night included singers Henrietta Adewole and Micky Dumoulin, dance duo Lauren and Terrell and puppet Patsy May.
Britain's Got Talent returns to UTV on Tuesday from 7.30pm.