United have already won FIFA's Club World Cup and English League Cup and are favourites to win the Premier League for the third successive season.

They are also in the semi-finals of the FA Cup and quarter-finals of the Champions League, but Ferguson said the unpredictable nature of cup football means that winning all five trophies is probably beyond them.

"The thing about cup football is you need to be the best but you also need a lot of luck and I think it's asking too much for all the games to go your way," he said in a wide-ranging interview in the New Statesman magazine.

"The one thing I will say is, this squad is the best I have ever had. Every game we play, I feel confident.

"At the moment, every attack fears our defence and every defence fears our midfield and attack. That gives you confidence but it is too tough a call."

In the interview, conducted by Alastair Campbell, a key advisor to former British Prime Minister Tony Blair, Ferguson said his greatest achievement was winning the treble of Champions League, Premier League and FA Cup in 1999.

He also admitted his biggest mistake was allowing Dutch defender Jaap Stam to leave the club and his biggest disappointment was not signing Paul Gascoigne when he left Newcastle United in 1988.

"He was a fabulous footballer and he would have done brilliantly here," he added of Gascogine.

Ferguson also said he has not yet decided on retirement.

"I am 67 now, my health is good, I still have the drive and the energy. I've been here for more than 22 years but I still get a buzz arriving at the training ground," he said.