Manchester United broke the club's transfer record to sign Spain midfielder Juan Mata from Chelsea for 37.1 million pounds on Saturday (UK time).
"I am thrilled to be joining United," said the 25-year-old Mata, who spent two years at Chelsea. "I have enjoyed some very happy years at Chelsea but the time has come for a new challenge."
United's previous record signing was Dimitar Berbatov, who joined for 30.7 million pounds from Tottenham in 2008.
United is banking on the arrival of Mata sparking an upturn in fortunes, with the team in seventh place in the English Premier League and struggling to qualify for next season's Champions League.
"United is the perfect place for me and I am excited at the chance I have to be part of the next phase in the club's history," Mata said. "Chelsea is a top club and I have many friends there but you cannot turn down the chance to join Manchester United."
United manager David Moyes said Mata will make his debut in the league game against Cardiff at Old Trafford on Tuesday.
"Juan is one of the finest playmakers in the game today and it's a real pleasure to have secured his signature," Moyes said. "He's been instrumental in Chelsea's recent success, with a notable 28 assists and 32 goals in his last two Premier League seasons.
"I have been impressed with the excitement he has shown to be part of the new Manchester United. I know our fans have admired Juan for some time now - he has always played well against us - and I know they will be delighted to see him in the red of United; starting on Tuesday."
Mata was Chelsea's player of the year for the last two seasons but fell out of favour following the return to Stamford Bridge of manager Jose Mourinho.
Mata played 135 times for Chelsea following his move from Spanish team Valencia in 2011.
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