He will help United through their injury crisis and came off the bench for the final 31 minutes of Sunday's
3-2 win at Manchester City
Scholes said: "I'm delighted the manager feels I can still make a contribution to the team."
The Salford-born player's return was announced only an hour before kick-off and two-goal hero Wayne Rooney revealed afterwards that the decision was a closely-guarded secret inside the club.
"It was a bit of a shock about Scholes - we didn't even know until we were in the dressing-room. But it's great to have such a quality player coming back," he told ITV Sport.
The veteran midfielder spent his entire career at Old Trafford and made his debut in the League Cup in 1994.
PAUL SCHOLES FACTFILE
16 November 1974, Salford
Man Utd debut:
21 September 1994, v Port Vale, League Cup (scores two goals)
28 May 2011, v Barcelona, Champions League final
Man Utd appearances (goals):
Man Utd honours:
10 Premier League titles; three FA Cups; two League Cups; five Community Shields; two Champions League titles
England caps (goals):
Scholes went on to make 676 appearances for United, winning 10 Premier League titles and two Champions League trophies.
The Premier League champions are beset by injury concerns in midfield, with Darren Fletcher ruled out for the rest of the season by a bowel condition and Tom Cleverley struggling with an ankle problem.
He added: "I've been pretty clear since I stopped playing that I miss it.
"I'm looking forward to playing my part in trying to bring more success to this great club."
United manager Sir Alex Ferguson feels Scholes, who quit international football in 2004 after winning 66 England caps, retired too soon.
"It's fantastic that Paul has made this decision," said Ferguson. "It's always sad to see great players end their careers, but especially so when they do it early.
Scholes looks back on his career
"But he has kept himself in great shape and I always felt that he had another season in him. It's terrific to have him back."
Ferguson has described Scholes as a "player of a lifetime" and admitted that replacing the midfielder would prove impossible.
He revealed he had been in talks with Scholes for several weeks about his return and is sure the midfielder can make an instant impact.
"It's been in the offing for a few weeks," Ferguson told ITV. "He has been playing really well. He came to see me and said he was missing it too much.
"There were no negatives as far as I'm concerned. The fans will be happy and I am happy.
Of players to attempt over 1,000 passes in the Premier League last season, Paul Scholes had the highest success rate (90.2%)
"Everyone knows he is one of the greatest players in Manchester United's history. He has not lost his real fitness."
Former United captain Roy Keane was shocked by the news but insists neither Ferguson nor Scholes have anything to lose.
He told ITV: "I'm very surprised, I have to say. I can understand that it's probably worth a gamble.
"But Scholesy retired seven or eight months ago because he thought his legs were gone and wasn't contributing in big games, so in that respect I'm surprised. But it's probably worth the gamble to lift the squad."
BBC pundit Robbie Savage believes Scholes, who turned down the chance to feature in the 2010 World Cup squad, could even force his way into Fabio Capello's Euro 2012 party.
He said: "What if Scholesy does well for another season - does he play for England? If he plays exceptionally well, then you never know."
Scholes will use the squad number 22 after Ashley Young took over his iconic number 18 at the start of the season.
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