South Yorkshire Police has been told it 'requires improvement' in how it legitimately keeps people safe and reduces crime.
The police watchdog, Her Majesty’s Inspectorate of Constabulary, branded the force 'good' last year, but in the latest inspection found standards had dropped.
It said the reduction in the rating was 'disappointing'.
The watchdog said a force is considered 'legitimate' if it has the 'consent of the public' and staff 'consistently behave in a way that is fair, ethical, and within the law'.
Inspectors look at how well forces treat people they serve and ensure staff act ethically and lawfully.
They also assess whether workforces are treated with fairness and respect.
This inspection also included, for the first time, an assessment of how well forces tackle the problem of abuse of authority for sexual gain by members of its workforce.
HMIC said that nationally 'this serious form of corruption betrays the trust of the public and, while we identified some examples of good practice, there remain significant areas where forces must improve'.
HM Inspector of Constabulary, Mike Cunningham, said: "South Yorkshire Police has been judged as 'requires improvement' in how legitimately it keeps people safe and reduces crime.
"This is disappointing as in 2015 we judged the force to be 'good'.
"Although treating people fairly is a stated priority for the force, recent changes in the way it operates are preventing it from both effective involvement with local communities and improving the morale of its workforce.
"An area the force is performing well in is how it ensures its workforce behaves ethically and lawfully. The force clarifies and reinforces acceptable behaviour standards, and also has a good understanding of the risks to its integrity.
"The force also recognises police abuse of authority for sexual gain as serious corruption and has highlighted this to the workforce and public.
"The force has systems to ensure the integrity of the organisation and its workforce is not compromised by inappropriate business interests or associations.
"The force vets new recruits effectively, but needs to ensure that the whole workforce is vetted to an appropriate standard in line with national guidance. "The force also needs to improve its understanding of workforce wellbeing issues. Although it has a number of methods to achieve this, we found that these are applied infrequently and inconsistently. It has developed a new personal assessment system, which is aimed at development and wellbeing, but it is too early to say if this will be successful.
"I will monitor closely this and the other areas HMIC has identified for improvement, and expect to see progress made."