SUNDERLAND midfielder Sean Thornton will spell out his desire to leave Wearside later today when he submits a written transfer request to the club's board.

The Republic of Ireland Under-21 international has become frustrated at his lack of first-team involvement this season, and has now accepted that he must sever his ties with the Black Cats.

Thornton's agent, Lee Payne, last night confirmed the midfielder was looking to leave the Stadium of Light, and revealed he would be putting his request in writing as soon as possible.

"We can confirm that Sean has asked for a transfer," said Payne. "And we will be confirming that in writing with the club this week.

"Sean loves everything about Sunderland, the club and especially the fans.

"But he has now reached a point where he must think about his career and his future. He is not happy with the current situation and something needs to change."

Thornton has made 32 starts for the Black Cats since making a £225,000 move from Tranmere in July 2002.

But, while he is widely acknowledged to be one of the club's best passers of a ball, the 21-year-old is yet to make a first-team start this season and was not even named on the bench for the recent wins over Gillingham and Nottingham Forest.

Summer signings Dean Whitehead and Liam Lawrence have both leapt ahead of Thornton in the midfield pecking order, and it is understood that concerns over the Irishman's fitness are limiting his first-team opportunities.

Everton were monitoring Thornton's situation last month but, with the Premiership transfer window having closed on August 31, a loan to another Championship side may be the only option until January.

Lawrence and Whitehead were both in Manchester yesterday - along with fellow newcomer Stephen Elliott - as a Football League tribunal met to determine how much Sunderland must pay for three of their summer additions.

The Black Cats will learn their fate this morning, with Mick McCarthy's future transfer plans currently hanging in the balance.

The Sunderland boss is still keen to sign another striker, despite the short-term acquisition of Simon Johnson, but all transfer activity will cease if the club are hit with a seven-figure bill.

Elliott's value is almost certain to have risen this season, as the former Manchester City striker has grabbed three Championship goals and become the Republic of Ireland Under-21s' leading goalscorer of all time.

Mansfield are confident of receiving more than £175,000 for Lawrence, as they turned down a bid of that size from Rotherham last season, and Oxford are demanding a similar amount for Whitehead.

Read more about Sunderland here.