Only about a third of postgraduates who are employed by their university as teachers feel that they receive appropriate supervision and feedback.
The statistic is included in a charter produced by the National Union of Students and the University and College Union setting out how universities should address the issue.
In a survey of about 350 postgraduates with teaching experience, 36 per cent agreed that they had received appropriate supervision and feedback, while 63 per cent had received no advice on professional development or training.
However, 87 per cent said they were confident that they performed their teaching role adequately.
The Postgraduate Employment Charter lists 10 principles of good practice relating to the employment of postgraduates.
Sally Hunt, UCU general secretary, said it "reaffirms that postgraduate researchers have the right to the same treatment as other university staff, meaning a proper contract, adequate payment, training and access to resources to support their work".
A spokeswoman for the NUS said the charter had been created in recognition of postgraduates' vulnerability to exploitation. But she said it was also necessary to ensure teaching quality.
"Some postgraduates take on quite a wide range of roles, including lecturing and pastoral support, as well as tutorials and seminars. As undergraduates rightly become more demanding ... there need to be processes in place to ensure postgraduates that teach are doing a decent job," she said.
One PhD candidate teaching at a post-1992 university, who did not want to be named, welcomed most of the charter's provisions but warned that insisting on parity between postgraduates and professional staff could discourage universities from employing PhD students or giving out teaching scholarships to fund PhDs.
"Many PhD students are postgraduate teaching assistants rather than lecturers and their role is intended specifically to help them to fund their PhDs and to give them valuable experience," she said.
"It would be a real shame if postgraduate teaching scholarships disappeared because universities could no longer afford them, because PhD students had been misclassified (as professionals)."
She added that, in her experience, doctoral students were often more committed to teaching than full-time lecturers, who "often see it as nothing more than a distraction from their 'real' work of research".