Darren McGrady, who worked for the Princess of Wales after her divorce from Prince Charles in 1996, accused Earl Spencer of neglecting the gravestone situated in the grounds of his Althorp Estate in Northamptonshire in England's East Midlands region.
The chef tweeted a series of images of the memorial, which is placed on an island in a lake, surrounded by overgrown vegetation and algae-covered water.
"If I cared for Princess Diana in life as you are doing in death I would have been fired," said McGrady, who also spent 11 years working at Buckingham Palace.
While Diana's final resting place was kept a secret for security reasons, a memorial including a small temple was built on the Althorp Estate which people can visit for 18 pounds.
McGrady, who now works in Texas and calls himself The Royal Chef on Twitter, added: "Made me so sad to see how Althorp Estate has neglected Princess Diana's resting place."
A spokesperson for the estate told The Telegraph the spread of algae was the result of a hot summer and was a common problem.
His criticism comes just a few weeks before the 17th anniversary of the People's Princess' tragic death.
