A Freedom of Information request by the Royal College of Midwives found that, of 49 institutions that responded, 41 do not adhere to the recommended ratio of one teacher to every 10 students.
The level was set by the English National Board for Nursing, Midwifery and Health Visiting in 1993 as a required standard for such courses, but the obligation was dropped when the Nursing and Midwifery Council took over their regulation in 2002.
Despite this, both the RCM and the NMC have retained the ratio as an informal guide to teaching quality.
The FoI request found that the average student-to-teacher ratio nationally rose from 13 to 13.6 students per teacher between the academic years 2009-10 and 2010-11.
Thames Valley University, now the University of West London, had the worst ratio, with 23.5 students to every teacher. At the other end of the scale, the University of the West of England had the best, with 5.8 students to every teacher.
The study also found that fewer new teachers were being recruited, with only 6 per cent of midwifery teachers under the age of 40. The RCM called this a "growing cause for concern".
Cathy Warwick, general secretary of the RCM, said there needed to be a "sea change" in the investment and support given to recruiting and retaining midwifery teachers.
"The future of midwifery will be shaped and determined by leadership in education and research today. Protecting and nurturing midwifery educators is a major challenge for the whole profession and one that we must all embrace," she said.
She also complained that there was a paucity of information about the state of midwifery courses, something that she said needed to be addressed with further research.