Ofsted is judging sixth-form colleges more harshly than schools, making it impossible for parents and students to draw fair comparisons, MPs have been warned.
Representatives from both FE and sixth-form colleges have complained that they are inspected under different criteria, even when they cover the same age range.
A submission to the Commons education select committee's investigation into the watchdog's role from the Sixth Form Colleges' Forum expressed its "concern and consternation with the differentiated standards which Ofsted uses".
Representatives from FE colleges also called for a "simple" and "transparent" system which would allow members of the public to make fair comparisons between neighbouring institutions which compete to attract students.
"The public perception, often shared by professionals who should know better, is that Ofsted judgments are transferable across sectors," the Sixth Form Colleges' Forum said.
"That is not the case. Ofsted uses differentiated judgments which set different standards for schools, FE colleges, other providers and sixth- form colleges."
The impact can be that sixth-form colleges are often judged more harshly than schools or FE colleges, the forum's submission said. "This can happen where institutions are in close geographical proximity and compete for students with each other.
"In an increasingly competitive environment it is vital that judgments are transparent, well understood and comparable across sectors".
The complaints follow Ofsted's criticism of the FE sector in the chief inspector's annual report last week. It said the number of outstanding colleges had fallen by two-thirds over the past year and criticised the quality of maths and science teaching for being "uninspiring".
Lesley Davies, assistant chief executive of the Association of Colleges, told MPs that the current system was "highly complex".
"We have different frameworks applying to the same age group," she said. "In schools they ask: `Are you happy and do you turn up on time?' and for colleges it's about outcomes and strategic leadership."
Ms Davies also told the committee that the 247 questions to be asked by inspectors were "far too many, it's burdensome on the inspectorate".
She added that some inspectors had an "over-reliance on data", meaning the reports were "not as full and rich as they used to be".
"We are not celebrating the good practice because we are too focused on risk assessment and value for money," she said.
Asha Khemka, principal of West Nottinghamshire College, said there was a need for "a system which is simple, which is transparent".
"We have got to come up with a framework which is absolutely giving the right information, the same information," she added.
A written submission by Staffordshire County Council said: "The inspection of school sixth-forms . does not receive the same level of scrutiny as inspections of other post-16 institutions, eg colleges, independent private providers."
With the outcomes of inspections being used to make "performance management" decisions, it is important that data is comparable, it added.
Ofsted declined to comment.