Several media outlets, including the BBC, claimed that Chelsea have accepted an offer worth around �37 million ($61.3 million, 45.3 million euros), which would exceed the �30.75 million that United paid Tottenham Hotspur for Dimitar Berbatov in 2008.

Mata, 25, was voted Chelsea's player of the year in his first two seasons at Stamford Bridge, but he has fallen down the pecking order following Jose Mourinho's return to the club as manager last year.

After United lost on penalties to Sunderland in the League Cup semi-finals at Old Trafford on Wednesday, the club's press officer warned journalists that Moyes would only answer questions on the match.

Pressed for an update on the situation as he left the press conference room, Moyes replied: "She just said, no questions."

Reports suggested that Mata has agreed a four-and-a-half-year contract at United and will undergo a medical examination on Thursday.

Chelsea have not acknowledged reports of United's offer, but Mata is believed to have missed a scheduled training session with the London club on Wednesday.

Prior to the Sunderland game, Moyes admitted that his club have not ruled out moving for players during the January transfer window.

Writing in the match-day programme, he said: "While we are actively scouting players all the time, a lot of the work we are doing now is preparation for the summer.

"If we can do any business now then we will certainly try to do it, but I will not compromise the standards required for a player who can come into this club and help improve it."

Moyes is bidding to arrest a startling decline in United's fortunes since he succeeded Alex Ferguson as manager last year.

The reigning English champions were also knocked out of the FA Cup in the third round by Swansea City earlier this month and are currently 14 points behind leaders Arsenal in the Premier League.