Manchester United will push through with a formal complaint to FIFA over Real Madrid's public pursuit of Cristiano Ronaldo.

The Mail on Sunday says United chief executive David Gill is intent on formalising the previously indirect attacks on Real Madrid president Ramon Calderon, by making a complaint to FIFA's disciplinary committee.

Though FIFA are almost powerless in these cases, the lodging of a complaint would further sour relations between the clubs in what has become an increasingly bitter battle that Calderon appears unable to control.

Gill will send a letter to FIFA tomorrow demanding the Spanish club end immediately their 'sustained campaign' to unsettle United's star player.

Patience at Old Trafford finally snapped after Calderon said his club were honoured that Ronaldo wanted to play for them.

A United source said: "Calderon's words were the straw that broke the camel's back. United didn't want to get FIFA involved but feel there is no other option now. David Gill has been away (for a meeting of Premier League chairmen) but he is angry about this. He'll be back on Monday and the first thing he's going to do is make our feelings official.

"The letter will say that Real Madrid are running a sustained campaign to try to unsettle our players to precipitate a transfer."