The trust said it was "extremely disappointing"
A West Yorkshire NHS trust says it will appeal against a rating given for the quality of its services in an annual review by the health watchdog.
The Mid Yorkshire Hospitals NHS Trust, which covers Dewsbury and Wakefield, was rated "good" last year but has now fallen to "fair".
It has improved from "fair" to "good" in its financial management.
Chief executive Julia Squire said it was "extremely disappointing" and the trust would appeal against the rating.
The trust is one of 12 others across Yorkshire and the Humber to have received a lower score for quality of services compared to the previous year's results by the Care Quality Commission (CQC).
Ms Squires said: "Our performance against the national standards and existing priorities and our self declaration against the core standards should have retained our good rating for quality of services.
"However, following a random inspection, the Care Quality Commission feels that we presented insufficient evidence for one of the core standards.
"This change to one standard reduced our rating to fair. Our intention is to appeal this decision."
Ms Squires added: "Our staff will be extremely disappointed as they have worked extremely hard to maintain and improve the quality of care and experience of Mid Yorkshire patients."
Elsewhere in the region Airedale NHS Trust has improved its quality of services score from "good" to "excellent".
Leeds Teaching Hospitals NHS Trust has improved from "weak" to "fair".
It is the fourth year the CQC has carried out its annual assessment of the NHS.
It is based on trusts' performances between 1 April 2008 and 31 March 2009 and rates each organisation as excellent, good, fair or weak on both its quality of services and financial management.