Simon Cowell is reported to have told the new X Factor judging panel they needed to ‘up their’ game after visiting auditions in London.

According to the Mirror Simon – who has been in the US for the past few months – has expressed fears that Kelly Rowland is dull and Tulisa Contostavlos is being too aggressive.

His remarks come following reports that the N-Dubz singer squared up to a singer at this week’s auditions and also hurled abuse at an audience member.

“Simon is at the auditions in London to see what the chemistry is like between the judges – and as always with Simon he’s had plenty to say,” an insider told the paper.

“He is worried that Tulisa is being too aggressive and needs to soften her comments sometimes if she wants to be liked by viewers.”

Simon also had worries Kelly may not connect with the British audience,” the source added, “and he wants to make sure she shines and is worthy of her place on the panel.

“Thankfully he knows what he gets with Louis Walsh and he has heard great things about Gary Barlow but he told all of them they need to raise their game collectively and then spoke to them individually about problems.”

According to sources the number of people taking part in auditions for this year’s series has fallen.