He said: "I've just not got the heart for it any more and I don't see the point in going through the motions.
"I've had a lot of knockbacks in my time but this has nothing to do with injury.
"I just don't enjoy it anymore."
The latest gossip and rumours from the newspapers.
Gareth Southgate will take charge of the England team for the remaining four matches of 2016.
Gareth Southgate has been placed in charge of England for their next four games after Sam Allardyce left by mutual
Celtic face another massive task in the Champions League when they welcome Manchester City to Parkhead on Wednesday night.
Following the departure of England manager Sam Allardyce, Under-21s coach Gareth Southgate is set to charge of the senior team
The latest gossip and rumours from the newspapers:The Daily Echo reports that Southampton defender JOSE FONTE, linked with a move