Heinze joined United from Paris St Germain in 2004

United disputed claims from Heinze, 29, who said the Red Devils gave him written permission to pursue a transfer to another club for a fee of £6.8m.

But a Premier League arbitration panel concluded that the permission "was unambiguous in that it envisages only an international transfer".

The defender is set to appeal against the panel's decision.

United will have 14 days to respond and, with the transfer window scheduled to shut on 31 August, Heinze is short of time if he wants to leave Old Trafford.

606: DEBATE Has Heinze made a massive mistake not only wanting to join our rivals but also going to arbitration?

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But his solicitor Richard Green says the Argentine could still join Liverpool after that date using Rule M4, which allows the Premier League discretion to grant a transfer outside the window.

Green told BBC Radio 5live: "My client and myself hoped the appeal would be concluded by the end of the window but that is clearly not going to take place.

"It is too early to say what may happen.

"I don't know if there will be any other teams interested. I don't know if United's attitude will change or if they are happy for him to stay or not."

The arbitration panel's verdict read: "The Premier League Board-appointed panel have heard submissions of evidence from both parties and has ruled to dismiss the player's case.

"The hearing concluded that nature and intention of the disputed 13 June 2007 letter, especially when taken in context of verbal discussions and Manchester United FC's transfer policy, was unambiguous in that it envisages only an international transfer.

"Furthermore, the hearing finds the letter constitutes an 'agreement to agree' and did not create an obligation or binding agreement for the club to transfer the player to any particular club."

United manager Sir Alex Ferguson and chief executive David Gill travelled to Monday's hearing in London to put their case and will be happy with the verdict.

A Manchester United spokesman said: "We are pleased that the panel has endorsed our case."

The Old Trafford side said they did not want to sell Heinze to a Premier League rival.

They said they made it clear to the defender both orally and in writing that he would not be allowed to join one of their title competitors.

But Green is adamant that Heinze should be allowed to move to Anfield, adding: "The player believes, as do his advisors, that the fax which was sent - which is the one being talked about - gives him the right to move to any club.

"The club were happy to sell the player.

"As a result his agent went to find clubs who were keen to buy him. That is how this has arisen - it is not because he has tried to force a sale or engineer a sale of any sort."

Transfers between the two north-west rivals are rare and the last first-team player to move from United to Liverpool was Phil Chisnall in 1964.

Any appeal will be heard by the Premier League's appeals committee.