The Welsh singer launched the third series of the BBC show with new judges Kylie Minogue and Ricky Wilson and stalwart Will.i.am.
Previous winners have failed to make their mark in the music world.
But Sir Tom said at yesterday's launch at London's New Broadcasting House that singers had already walked through one door by getting on to the show.
He said: "There's no guarantee in showbusiness. It's one step at a time. It's always down to the public whether that person is going to be a star."
"There's no guarantee that if you win this, you will be a star."
In an apparent dig at The X Factor, Wilson, lead singer with Kaiser Chiefs, added: "If you want to see people making fools of themselves you can go to other stations."
Minogue, who was approached to join the first series but turned it down, said that she was "really comfortable" being the only woman on the panel because it "stops any stories about bitchiness and cattiness."
She paid tribute to her predecessor Jessie J, praising her voice and character and adding: "I hope Jessie enjoys seeing me in her seat."