The Chief Inspector of Constabulary has described the decision to remove Chief Constable David Crompton from his post as 'conspicuously unfair, disproportionate and unreasonable'.
Sir Tom Winsor's views were expressed in correspondence with South Yorkshire's Police and Crime Commissioner, Alan Billings, while was debating the police chief's future.
The PCC called for Mr Crompton's immediate resignation yesterday over a statement he made following the conclusion of the Hillsborough disaster inquests.
He said Mr Crompton had 'sought to justify' the stance taken by his force's lawyers during the inquests, which questioned the behaviour of fans leading up to the crush which claimed 96 lives in 1989 - despite the inquests clearing supporters of any blame.
He said it was 'insensitive' and 'indicated that the leadership of South Yorkshire Police had not learnt the lessons of past failures'.
Mr Crompton is set to challenge the PCC's decision in court.
He is backed by Sir Tom, who in a letter to the PCC said the 'forced removal' of Mr Crompton was 'conspicuously unfair, disproportionate and so unreasonable that I cannot understand how the PCC has reached this view'.
He said the reasons came 'nowhere close to the sort of concerns' he would expect for the removal of a Chief Constable.
Sir Tom's advice to the PCC was published in full after his decision was announced.
Stephen Watson has been appointed as the the successor for Mr Crompton, who was due to retired in November.
He is running the force on a temporary basis.