Ferguson has hit back at Rafa Benitez's comments

Heinze left Old Trafford for Real Madrid, but only after a protracted attempt to join Liverpool which went to a Premier League arbitration panel.

Reds boss Rafael Benitez was unhappy with the three-man panel's verdict.

But United boss Ferguson said: "We are looking at Liverpool's whole role in this. We are not letting them off with it. That is what we are investigating."

Heinze argued that a letter signed by Red Devils chief executive David Gill - citing a fee at which a deal could be done - constituted an agreement to sell.

But the Premier League ruled in United's favour, ending any hope of them selling to rivals Liverpool. Heinze was instead sold to Spanish side Real.

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Benitez said he was 100% confident that Heinze would be allowed to move to Anfield and hit out at the Premier League and United after the ruling.

Ferguson labelled Benitez's remarks "churlish", then confirmed Gill is leading an inquiry into the way Liverpool have acted.

Asked whether he was alleging Liverpool had 'tapped up' Heinze, Ferguson replied: "We will see how it comes out. David Gill is working on that at the moment. We will get to the bottom of it."

Ferguson was also quick to hit back at Benitez's jibe that United enjoy favouritism in the drafting of the fixture list.

"It is disappointing stuff," said Ferguson. "He talks about the fixtures. He should have seen our fixtures over the last two or three seasons.

"There was never any chance of Liverpool winning that case, so Rafa clearly did not read the Premier League rules. It is just churlish."

I want to ask why it was so difficult for Liverpool to sign Javier Mascherano but it was so easy for Carlos Tevez to join United Rafa Benitez

Benitez earlier said: "He has a document which is clear but the league prefers to believe the word of someone who made a mistake.

"I know there were accusations made against Liverpool in the hearing which were unbelievable. How can this be allowed?"

He also expressed his anger at the Premier League's fixture scheduling ahead of the club's away game at Sunderland at 1245 BST this Saturday.

The Spaniard said: "I would like to ask the Premier League why is it that Liverpool always plays the most fixtures away from home in an early kick-off, following an international break?

"We had more than the top clubs last season and we have four already to prepare for this season."

Benitez was also angry about what he claimed was the drawn-out nature of Liverpool's signing of Javier Mascherano, compared to United's signing of Carlos Tevez - a deal he felt was pushed through quicker.

Mascherano and Tevez were signed last summer by West Ham but their moves were complicated over issues relating to third-party ownership in their contracts.

"I want to ask the Premier League why it was so difficult for Liverpool to sign Javier Mascherano, when we had to wait a long time for the paperwork, but it was so easy for Carlos Tevez to join Manchester United."