Wardle said: "This has not been an easy decision to make, as I have enjoyed every minute of my time at Manchester City in whatever role I have been asked to do.
"I am proud of the fact that during my time as Chairman, we have never dropped out of the top flight and that we achieved the stability that the Club and its fans had been craving for so long.
"It was for this reason that I took the necessary step of selling the Club last year, as I believed we had taken Manchester City as far as it could go.
"We had a wonderful stadium, top class training facilities and a fantastic Academy system, however it was obvious that further investment was needed.
"Now, with all these things in place and together with Dr Thaksin's investment, I truly believe that the Club is set up for a very positive future. We have one of the brightest managers in the game and a great crop of exciting players."