Sheffield Wednesday say "outstanding legal issues" remain regarding Steve Bruce's departure from the club.
The Championship side insist they're "disappointed" to learn he was confirmed as Newcastle's new head coach this morning.
Bruce resigned from his previous job with the Yorkshire team - along with coaches Steve Agnew and Stephen Clemence.
Sheffield Wednesday say they're now seeking legal advice.
Sheffield Wednesday have issued the following club statement #swfc
— Sheffield Wednesday (@swfc) July 17, 2019
“The club is disappointed to learn via public statement issued by Newcastle United that it has appointed former Sheffield Wednesday staff Steve Bruce, Steve Agnew and Steve Clemence.
“Regardless of the fact that the staff resigned from their positions with the Club on Monday, there remains outstanding legal issues to be resolved between the club and the staff and Newcastle United.
“The Club is currently considering its position and taking the appropriate legal advice.
“No further comment will be made by the Club concerning this matter.”
Bruce had earlier spoken of his delight at succeeding Rafa Benitez at Saint James' Park.
"I'm delighted and incredibly proud to be appointed as head coach of Newcastle United.
"This is my boyhood club and it was my dad's club, so this is a very special moment for me and my family.
"There is a huge challenge ahead of us, but it's one that my staff and I are ready for.
"We'll roll our sleeves up and we'll be giving it everything from the off to ensure supporters have a successful team that they can be proud of."