After the referendum in June last year, EU nurses joining the Nursing and Midwifery Council (NMC) register fell to fewer than 200 per month compared to nearly 800 a month for the same period in 2015.
A separate FOI request has revealed that 2,700 EU nurses already working in the UK left the NMC register in 2016.
Janet Davies, RCN Chief Executive & General Secretary, said: “EU nationals working in the NHS need a clear signal from Theresa May that they are wanted and welcome to stay. Her failure to guarantee their right to remain is leaving soaring numbers heading for the door. Few are able to live with such uncertainty.
“The Government is turning off the supply of qualified nurses from around the world at the very moment the health service is in a staffing crisis like never before. They cannot afford to lose the international workers the NHS relies on.”
NHS hospitals and community services have increasingly relied on international recruitment due to a shortage of British nurses. The number of EU nurses available trebled between 2011 and 2016.
It is expected that that one in three nurses will retire in the next 10 years and 24,000 nursing posts are currently unfilled in England.
Prime Minister Theresa May will trigger Article 50 on 29 March.