The TV judge, 63, was taken on stage and had a hood put over his head by a magician, who was masked during the show.
The Mirror reported that lighter fluid was poured all over the hood, then his head was locked in a box which was opened at the front for the audience to see inside.
The magician then dropped a flame into the box and his entire head went up in flames, leaving the other judges and audience in shock.
One onlooker said: ‘The audience screamed, judges were out of their seats screaming, everyone was totally stunned. The flames were put out and Simon got up looking a bit shell-shocked while the rest of the judges breathed a sigh of relief as he returned to the panel.’
Afterwards Simon quipped: ‘I could feel liquid being poured on me but I had no idea it was lighter fluid!’
Fellow judge Alesha Dixon added: ‘Simon, he set your entire head on fire! It was very scary, you are a very brave man to do that. It made me realise how much we care about you, I was ready to come up there and rescue you.’
And new judge Bruno Tonioli said: ‘It reminded me of a scene from that movie, Saw, seeing Simon’s head in that contraption. I was captivated, I’ve never seen anything like that.’
It comes after Bruno reportedly threw Britain’s Got Talent auditions into chaos when he pressed his Golden Buzzer twice before admitting he’d never watched the show.
The former Strictly judge, 67, has joined the panel of the ITV talent competition for as a replacement for comedian David Walliams , 51, who left after he made ‘disrespectful’ comments about contestants from previous series.
Professional dancer Bruno apparently did not realise that the Golden Buzzer, which automatically sends auditionees through to the live finals when pressed, could only be used once, and reacted when the audience began cheering for an impressive act.
Producers are said to have panicked after they were left with a decision of whether to send the hopeful through or cancel Bruno’s second buzz.
A source told The Sun : ‘It was madness. The buzzer worked but there was no glitter in the cannon above the stage as it had already been used up.
‘Then there were discussions with the top execs and Simon [Cowell], who had his head in his hands.’
The source said Bruno then admitted he had never actually watched Britain’s Got Talent before, claiming he did not want to be influenced by the judging styles from previous series.
Luckily, the audience saw the funny side and Bruno is remaining on the panel as the new big money signing.