Research that has revealed wide disparities in PhD vivas has prompted calls for "common ground rules" so that students are better prepared for the examinations.
Guidelines would particularly benefit female students, who find the prospect of face-to-face questioning of a PhD thesis much more alarming than their male counterparts, according to the study.
"The data show that some students are disappointed by their mock and real vivas," said James Hartley, professor of psychology at Keele University, who co-authored the study.
"Some expect a grilling and feel they are not tested. Some expect a scholarly debate and feel short-changed. Some feel that they can't get a word in edgeways and some feel that they are being attacked.
"Our data suggest that some get very anxious and stressed before the examination and it is here that the stress lies rather than in the viva itself. The viva becomes a bit of a letdown."
The study, co-authored by Claire Fox, who is also at Keele, focused in particular on mock vivas - finding "considerable disparities in procedures within and across institutions" in a survey of 29 postgraduates across the UK.
Although 90 per cent of those surveyed said that mock vivas were "helpful", the study concludes that there are often substantial differences between preparatory sessions and the real viva.
The academics note that a common criticism was that mock vivas did not ask students "important" questions about their thesis, which they then faced in the real one.
Professor Hartley and Dr Fox conclude that "if mock vivas are to be held, they need to be treated seriously, and common ground rules need to be established at least within departments".
The survey also found a "significant" difference between how nervous male and female postgraduates felt before a mock viva.
The paper in the journal Studies in Higher Education says: "The summary results suggest that the majority of the candidates were apprehensive to some degree and that the women full-time respondents felt more anxious than the men before their mock vivas."
Two of the women who responded described feeling "scared" and "terrified".
Other studies have also shown that female students are more anxious about examinations than men.
The academics urge departments to consider allowing mock vivas to run for one hour and to involve two members of staff, to ensure that students are given feedback before their real viva.
But they also conclude that students need to be better prepared for both their mock and real vivas.
Some disciplines, such as psychology, have guidelines for PhD examiners that are given to students before their vivas.
Jim Ewing, general secretary of the National Postgraduate Committee, said that the viva was intended as an oral examination to legitimate the thesis and was not supposed to be a "hatchet job" by the external examiner on the basic thesis proposal.
"We have heard horror stories from candidates who have been assured that their theses are fine, only to be faced with major revisions," he said.
"We need clear, precise, well-publicised nationally accepted guidelines on what the candidate has a right to expect and a duty to perform in the viva, so that, as well as coming up to scratch, the candidate knows when to call foul."
The study surveyed students from 20 universities, working in the arts, social sciences and physical sciences.