He was director of Finance and Property Services, HM Household until 1996 then became the Queen's treasurer or Keeper of the Privy Purse from 1996-2002.
During his time with the monarch he performed what many saw as a vital modernising role in slashing royal expenditure before becoming Charles's private secretary in 2002.
In 2005 he became the prince's principal private secretary.
A Clarence House spokeswoman said Sir Michael would continue to undertake investment and financial projects for the prince and provide advice on a part-time basis.
She added: 'The Prince of Wales and the Duchess of Cornwall fully understand Sir Michael's wish to return to the private sector, are hugely grateful to him for all he has done and wish him every success in the future.'
During his time with the monarch he performed what many saw as a vital modernising role in slashing royal expenditure before becoming Charles's private secretary in 2002.
In 2005 he became the prince's principal private secretary.
A Clarence House spokeswoman said Sir Michael would continue to undertake investment and financial projects for the prince and provide advice on a part-time basis.
She added: 'The Prince of Wales and the Duchess of Cornwall fully understand Sir Michael's wish to return to the private sector, are hugely grateful to him for all he has done and wish him every success in the future.'