Just 1% of UK medical students come from unskilled backgrounds, according to a report.
Most new medical students are now female
Figures from the British Medical Association show most of those who study medicine still come from professional or managerial backgrounds.
The report calls on universities to take action to reduce this gap.
"Medicine must not be the preserve of the middle classes," said Dr Vivienne Nathanson, head of science and ethics at the BMA.
According to the report, students from professional backgrounds have been twice as likely to be accepted on a medical degree course than those from unskilled backgrounds in recent years.
"In fact, the likelihood of an applicant being accepted to medical school generally declines with socio-economic background," the report says.
However, it acknowledges that there are wide variations between different universities.
It says much of this is due to differences in the way students are selected. It says some universities may make it harder for students from disadvantaged backgrounds to get into medical school.
It calls for schools with low numbers of these students to be investigated.
"Differential acceptance rates should prompt a thorough investigation of the selection processes for medical school," it says.
"Even if there is no direct discrimination against students with lower socio-economic backgrounds, it is still likely that these applicants experience a certain degree of disadvantage during the application and selection process."
The report warns that government plans to introduce university tuition fees could make it more difficult to attract students from unskilled backgrounds.
It also reveals how the medical workforce is set to change in the years ahead.
In 2003, over 60% of all new medical students were women. In the past, most were men. By 2012 women doctors could outnumber men.
Overall, one in three come from ethnic minorities - most of these come from Asian backgrounds.
It says more needs to be done to attract students from other ethnic minorities.
The report calls on medical schools to monitor the backgrounds of students.
"At the moment, very few medical schools are systematically monitoring their application and admissions statistics and making changes based on their findings," it says.
Dr Peter Dangerfield, chairman of the BMA's board of medical education, said it was important that students in medical schools were reflective of the wider population.
"The NHS needs doctors who can relate to all their patients, but at the moment the profession is far from being representative of the public it serves.
"Medical schools need to guard against any kind of discrimination - intentional or otherwise."
Professor David Gordon, chairman of the Council of Heads of Medical Schools, said universities were trying to ensure people from diverse backgrounds studied medicine.
"I think it is wrong to say some medical schools are not doing enough. They are working very hard indeed at widening participation," he told BBC News Online.
Liberal Democrat education spokesman Phil Willis used the report to call on the government to scrap tuition fees for long courses like medicine.
"It is crucial that the government accepts the Liberal Democrat amendment that all courses over four years should be fee exempt.
"Without that first step the damage to the medical profession will be permanently done."