The Cornwall Partnership Trust, which deals with mental health care in the county, has lost its appeal to regain its three star rating. The Healthcare Commission considered an appeal from the Trust, but decided against giving it back its third star.
Two stars in the performance rating show the Trust is performing well.
But it means the Trust no longer qualifies to become the first Foundation Trust in the country. Three stars are needed for it to apply.
The Trust says it is considering meeting with the Healthcare Commission to discuss the rating.
Foundation status gives NHS trusts more freedom over their finances and the way they develop services.