Manchester United boss Sir Alex Ferguson says Barcelona's recent slump is a wake-up call for his players.

United head to Newcastle in a seemingly impregnable position at the Premier League summit.

But it is only a matter of weeks since Barcelona were in a similar position. Now the gap to Real Madrid is down to four and Barca are suddenly wobbling.

"Barcelona have been playing Saturday-Wednesday-Saturday since January," said Ferguson.

"They did have an 11-point lead, now it is four and for the first time this season they lost a game after being in front.

"It is a warning, and there are plenty of them, that if you take your foot off the pedal you can get a kick in the teeth."