Martin O'Neill has resigned as Aston Villa manager, effective immediately. No reason has yet been given for O'Neill's dramatic decision to leave after four years in the job.

Meanwhile first team coach Kevin MacDonald has been appointed caretaker boss and will prepare the side for their Premier League opener against West Ham on Saturday.

In a statement O'Neill, appointed in 2006, wrote: 'I have enjoyed my time at Aston Villa immensely. It's obviously a wrench to be leaving such a magnificent club.'

The 58-year-old, who led Villa to their third successive sixth-placed finish in the Premier League last season, added: '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.'

O'Neill has been widely credited with doing a great job at Villa taking over from the demoralized side that David O'Leary left.

O'Neill transformed the fortunes of Villa, culminating in an appearance in the final of last season's Carling Cup, their first Wembley final for 10 years.

It is not known yet whether the response O'Neill received from the club's hierarchy led to his decision to end his four-year reign at the Midlands club.

Villa chairman and and chief executive Paul Faulkner said of O'Neill's departure: 'The club would like to thank Martin for the great work he has done at Aston Villa over the past four years. He has helped to establish the club in the upper echelons of the Premier League, has taken us to Wembley and we have also qualified for European competition for the past three seasons under his management. We wish him the best in the future.'