It is believed that O'Neill is frustrated at the imminent sale of star midfielder James Milner to Manchester City.

Last season, Villa lost sacrificed another key player in Gareth Barry, who also joined the big-spending Citizens.

Villa have yet to explain the shocking decision of O'Neill, who also refused to comment.

"I have enjoyed my time at Aston Villa immensely. It's obviously a wrench to be leaving such a magnificent club," O'Neill told Villa's official site.

"I would like to pay tribute to the Villa players, my coaching staff and the Villa supporters for all the support and encouragement they have given both the club and me personally during my time as manager."