The number of child and adolescent psychiatrists has fallen by 6.3% in England over the past four years, their professional body has warned.
The Royal College of Psychiatrists is urging the government to allow recruitment from overseas in order to meet the four-week waiting time target it has planned.
Kent, Surrey and Sussex have only 3.75 child and adolescent psychiatrists per 100,000 children, the college said.
In London, the tally is around 17.
To increase numbers, the college wants NHS child and adolescent psychiatrists to be added to the shortage occupation list, run by the Home Office.
Professions on this list have priority getting visas to employ staff from outside Europe because they are struggling to recruit at home.
Old age psychiatrists are currently on the shortage list but child and adolescent psychiatrists are not.
Dr Bernadka Dubicka, chair of the Royal College of Psychiatrists' child and adolescent faculty said: "Earlier this year an Indian child and adolescent psychiatrist had their visa application rejected because the quota for Tier 2 visas had been reached and they did not qualify as a priority on the shortage specialty list.
"Short staffing in child and adolescent mental health services is no secret."
She said the government's own green paper on children and young people's mental health predicted a rise in referrals, and recruiting from overseas was the key to employing qualified doctors more quickly.