The First Division club will now be seeking compensation after Cotterill quit as manager of the Potteries club on Thursday to become assistant manager at the Stadium of Light.

He had joined Stoke on a three-year deal in May 2002 and it is thought there was a clause in his contract allowing him to leave.

But Gislason said Stoke would be seeking compensation and he told BBC Radio Five Live Sunderland contacted Cotterill without informing him first.

The Stoke chairman also claims he only got a call from his Black Cats counterpart Bob Murray shortly before the news conference unveiling Cotterill and new Sunderland manager Howard Wilkinson.

"I expressed my disappointment in the way this whole matter had been conducted," said Gislason of his conversation with Murray.

"I feel this is not a proper way to approach things.

"There's no doubt about that," answered Gislason when asked whether he thought Cotterill had been illegally approached.

He added: "He [Cotterill] himself said the club spoke to him on Tuesday and he had already accepted when he called me."

The Football Association revealed it would be seeking compensation from Sunderland too because Wilkinson was still under contract.