The singer, who will judge the show alongside Will Young, Will.i.am and Tom Jones, said: “No one wants to know if you have a sore throat — that’s how hard I am going to be on the contestants.”
Jessie is backing girlband Little Mix to beat Marcus Collins and Amelia Lily to win The X Factor this weekend.
She said: “Little Mix are cool girls.
“It’s nice to hang out with them and make sure that they realise that there is a world beyond winning and there’s a world beyond X Factor.”
BLACK Eyed Peas frontman Will.i.am will also head up the show, after finally agreeing a deal worth £500,000.
The US rapper will join Jessie J, Tom Jones and either Will Young or Kaiser Chiefs’ Ricky Wilson on the panel next year.
Will.i.am said last night: “I’m proud to be doing The Voice UK because the UK was the first place I saw success.”
The BBC are under fire for splashing out on the show as other departments are cut.