Graduates' degree classes are not comparable between universities or subjects, the UK's higher education quality watchdog has said.
A general review of degree classifications is underway
Universities' different marking methods make it difficult for employers to know what to make of someone's degree class.
A Quality Assurance Agency (QAA) report questions whether students with the same marks are equally able or have worked equally hard.
The agency called for a "more reliable, robust, fair and transparent" system.
The QAA report, The classification of degree awards, is the second in a series and is based on the audits the agency carried out at 128 universities and colleges of higher education between 2003 and 2006.
Its previous report proposed that the system of first, 2:1, 2:2 and third class honours degrees be replaced by distinction, pass and fail along with more detailed transcripts of achievement.
That was aimed at assisting the ongoing Burgess Inquiry into the degree classification system in England, which is expected to report soon.
The latest study looks at how higher education institutions measure and certify their students' achievements.
It said the reliability of these measurements should allow students to judge their success and may determine their employability.
It should enable employers to make recruitment decisions and give institutions themselves key information to assess their success.
But "an appearance of inconsistency and unfairness arises" because of an assumption that students who obtain similar marks in parts of their studies should merit the same overall judgement.
The problem was that this overlooks the fact that marking practices vary, it said.
% FIRSTS AWARDED, 2006
Mathematical sciences 26.2%
Physical sciences 18.3%
Engineering & technology 18.2%
Computer science 13.2%
Historical & philosophical studies 12.7%
Creative arts & design 12.5%
Subjects allied to medicine 11.7%
Biological sciences 11.2%
Agriculture & related subjects 11.0%
Architecture, building & planning 9.4%
Social studies 9.0%
Mass communications & documentation 7.5%
Business & administrative studies 7.3%
Medicine & dentistry 3.4%
Veterinary science 1.5%
Source: Higher Education Statistics Agency
In particular, it was "questionable" whether students of different subjects who have obtained the same marks are equally able or have worked equally hard, the report said.
In general, institutions "have only weak control over the marking practices of examiners".
Reliable data on their behaviour was not systematically collected or analysed, it added.
"It is, however, acknowledged that in some subjects marks within 10% of the maximum obtainable are not uncommon, while in other subjects such marks are never given, and yet in others, practice will be changing, sometimes slowly, sometimes rapidly."
The issue was further complicated where decisions on degree classifications are delegated to external assessment boards.
And boards of examiners were granted different levels of discretion from one place to another.
"In one report it was noted that discretion 'may include discounting the lowest module mark, provided that the mark indicates that a pass standard has been achieved'."
Another audit report noted there was no institution-wide guidance or monitoring of the use of such discretion.
The report said students on joint honours courses suffered the most from inconsistencies.
The QAA's chief executive, Peter Williams, said its report "should act as a call to action to ensure that future measures of students' achievements are more reliable, robust, fair and transparent than the current arrangements".
He told BBC News that degree classifications told employers nothing.
They were a "proxy index" of some sort but too vague and misleading to be of any use.
A spokesperson for the vice-chancellors' group Universities UK said: "The report demonstrates the need to provide more information about student achievement which is why the findings of the Burgess Review on the classification of degree results will be important."