Sunderland and Tranmere Rovers are at odds over Republic of Ireland Under-19 international Sean Thornton.
The Wearsiders today confirmed they are in talks over a compensation package as they try to lure the 18-year-old, who is out of contract in June, to the Stadium of Light.
But Tranmere have reacted furiously to the midfielder's claims he has trained and agreed a five-year deal with Sunderland.
Rovers have accused Sunderland, who have been talking to Thornton for several months, of making an illegal approach and say they will report them to the Football Association and Football League.
"It seems Sunderland have tried to snatch our player by making an illegal approach," manager Dave Watson told the club's website. "Both Sean and Sunderland have shown a complete lack of respect for Tranmere Rovers and the FA and Football League rules, which is totally out of order.
"This is a dreadful way for Sean to treat Tranmere after the opportunity we have given him and the record-breaking contract we have been discussing with him".
Tranmere chairperson Lorraine Rogers said: "If Sunderland want Sean Thornton, they will have to pay compensation. We have a very strong case and will let a tribunal decide if necessary".
PA