Manchester United boss Sir Alex Ferguson admits Manchester City are now his biggest challenge.

All-conquering Fergie says Roberto Mancini's Etihad outfit will be handled in the same way as Liverpool, Arsenal and Chelsea.

Ferguson said: "Challenges are what we are made of. We face them head-on, we don't run from them.

"I've been lucky I've been involved in great competitions with teams like Liverpool, Arsenal and Chelsea.

"It's fantastic. No dull moments.

"City are our biggest threat now, there's no question about that and we are their biggest threat.

"The great thing about football is that things can change and their fortunes changed when Sheikh Mansour took over.

"I knew the minute that happened it was going to be a different ball game."