A "significant number" of associate inspectors and a few lead inspectors do not possess the skills and experience required to pass judgment on the effectiveness of colleges, a report says.
The allegations are made in a submission from the Association of Colleges, to the education and skills' select committee which has listed 16 key points for MPs to consider.
Judith Norrington, AoC director of curriculum and quality, said: "It appears that from speaking to our members that their experience of inspection quality is varied. Some are extremely well conducted by experienced inspectors and other inspections have sparked concern.
"We also know that there are some areas that are not assessed because there are not enough qualified people to carry out an inspection of that particular area."
The AoC also told the committee: "We are concerned that the criteria for the grades for leadership and management appear sometimes to be inconsistently applied.
"Colleges should be judged with reference to the performance of similar colleges and not against those with a very different profile."
The committee will be taking formal oral evidence from David Bell, the chief inspector of schools, on Monday in its biannual scrutiny of the Office for Standards in Education.
In written evidence, the AoC says that the standards of inspectors have given colleges cause for concern, and that in some areas there have not been enough inspectors to carry out the job.
The report adds: "The large and vital contribution made by many colleges to widening participation, combating social exclusion and delivering the objectives of Success for All, is rarely reflected in the grades for such colleges.
"The measures for judging successful outcomes of student learning in further education are too crude. It is vital that ways of measuring the 'value added' to all learners are introduced."
It adds: "The time is right to question whether inspection in its current intensive and costly form is an effective way of making further improvements."