The singer has been accused of being 'a homewrecker' by mother-of-one Stephanie Ward, who spoke out about the alleged affair between them.
The X Factor judge denied it on Twitter and said: "Don't want 2 get in 2 a public war of words, but what is being written about me 2moro is absolutely not true."
She continued: "I av always kept it real n that aint about 2 change, u r a liar n a psychotic obsessed nutta! wait til I tell it how it is... wrongen."
Tulisa's lawyers have issued a statement saying: "Tulisa categorically denies any allegation that she entered any relationship with Danny Simpson while he was still in a relationship with Victoria Ward, or that she was aware of Victoria Ward's claim to be pregnant with Danny Simpson's child.
"She has instructed her lawyers to deal firmly with all those who have published those allegations to ensure that they are retracted."
Four months pregnant Stephanie, who is carrying their second child, said: "He's not famous and nobody knows who he is, but Tulisa is obviously a homewrecker.
"She knows about me and the baby because when she called him to let him know the story was breaking, he told me she said to him, 'I'm just letting you know now there's a story breaking as I know you've got a baby on the way and I know you won't want your girlfriend to see it'.
"It's fair enough if a girl doesn't know about it, but the fact that she knows I've got a baby on the way and another child is bad."
She continued to The Sun: "I don't want anything more to do with him now. He's made me feel like absolute s**t.
"He was upset and saying sorry and saying it was nothing, that he just went for food with her and his friends."
"He admitted he has been texting her. I said it's obviously more than just food with friends, and he told me they got on well.
"So I asked if he'd slept with her but he said no it was just food.
"He kept just saying, 'I'm sorry'. I have a law degree and I gave up my career as a solicitor to have children with him."
Danny has also been linked to Natasha Giggs, Imogen Thomas and Roxanne Pallett.
Tulisa is thought to have said on the subject: "I'm not going to sell a story on myself. I will say something when I am ready."
Newcastle United star Danny has not commented on it yet, but has been backing Tulisa on The X Factor.
He wrote recently about the spat between Barlow and her: "Wow Gary going in on Tulisa, that's below the belt for live tv lol."
And: "I liked that version of Tinie's song that @Ella__Henderson just did. The girl's 16 years old n she's just absolutely smashed it."