There are not enough NHS psychiatrists in some parts of the UK and more should be recruited to improve care, says the Royal College of Psychiatrists.
In Scotland, there are 10 consultant psychiatrists per 100,000 people – but only eight for the same number in England and Northern Ireland, and just six in Wales.
London’s tally is more than double that of the east of England.
English health ministers said thousands of new posts were already planned.
The government in England recently announced there would be 570 extra consultant psychiatrists by 2020-21.
And the Welsh government said it was committed to improving mental health services in Wales.
But the Royal College of Psychiatrists said there were too few medical students specialising in psychiatry and not enough psychiatrists becoming consultants.
In England, in the past five years, it said there had been a 1.7% increase in psychiatry consultants compared with a 20.2% increase in other consultants across the NHS.
Areas such as London and the north-east of England have more than 11 psychiatrists per 100,000 people but the south-west has six and Yorkshire five.
Prof Wendy Burn, President of the Royal College of Psychiatrists, said: “People with a severe mental illness should expect to see a specialist consultant, just as you would for a severe physical illness.
“The huge variation in consultant psychiatrists across the country means reality is increasingly falling short of our expectations.
“As highly skilled medics, psychiatrists must be able to spot the nuance in symptoms, ask the right questions, and understand what the problem is.
“Without psychiatrists to lead specialist mental health teams we cannot deliver the high-quality care that our patients deserve.”
Health ministers in England said: “We want people with mental health conditions to receive better treatment and that means having the right NHS staff.
“We have started one of the biggest expansions of mental health services in Europe with our ambition to create 21,000 new posts by 2021 by supporting those already in the profession to stay and giving incentives to those considering a career in mental health.”
A Welsh government spokesperson said it recognised there was a recruitment challenge in psychiatry in the UK.
“We are committed to the continuous improvement of access to mental health services in Wales.”