One in 10 consultant psychiatrist roles is currently unfilled in NHS organisations in England, says a report by the Royal College of Psychiatrists.
It says the number of unfilled posts has doubled in the past four years.
Wales is also struggling to fill posts, with vacancies of 9%, while Scotland and Northern Ireland have vacancy rates of 6% and 2% respectively.
The college called the vacancies "frankly alarming" and said they increased waiting times for patients.
Prof Wendy Burn, from the Royal College of Psychiatrists, said the current situation meant patients might be waiting months to see a psychiatrist, during which time they could be getting worse.
She said it was "a scandal" because if you had cancer you would expect to see a cancer specialist.
"Patients won't get the care they need. Some will have been plucking up the courage to go to their GP, and then there is no-one to see them when they are referred," she said.
Prof Burn said the rise in vacancies was down to the difficulty in recruiting psychiatrists.
At the same time, more posts were being created for consultant psychiatrists as mental illness moved up the health agenda, but there were no specialists to fill them, she said.
She said medical schools need to broaden their pool of applicants in order to get more psychiatrists into the workplace.
"We are keen that medical schools should take in people studying psychology A-level – because they will be more likely to end up as psychiatrists."
It takes 13 years to train as a consultant psychiatrist.
The report found the situation was worst for psychiatrists in England who specialise in treating children or older people.
In both specialities, the vacancy rates doubled from roughly 6% in 2013 to 12% in 2017.
The findings are supported by a recent review by the Care Quality Commission which found young people are facing long waiting times and unequal access to mental health services.
Saffron Cordery, from NHS Providers, said the shortages were "deeply worrying".
"The government's laudable ambition to improve mental health services will only be realised if we have the right workforce with the right skills in the right place."
The Welsh Government said there was a need for more doctors to choose psychiatry as a specialty.
It said it was spending more on mental health services than any other part of the NHS.