Nursing leaders have expressed "major concern" as rising numbers of nurses from the EU continue to leave the UK.
New figures from the Nursing and Midwifery Council (NMC) - which maintains a register of nurses practising in the UK as part of its regulatory duties - show a significant rise in the number of nurses from Europe leaving the British register and a plummeting number applying to join.
The NMC said in 2017/18, 3,962 nurses from the EU left the register, an increase of 29% from the previous year.
The number of nurses from the European Economic Area (EEA) who have left Britain has been steadily increasing from 2013/14.
Meanwhile the number of European nurses who joined the register in 2017/18 dropped by 87% compared with the previous year, according to a new report from the NMC.
In the last financial year, 805 EU nurses and midwives joined the register compared with 6,382 the year before.
A survey of 227 European nurses who had left the register found 47% said Brexit had encouraged them to consider work outside of the UK.
The NMC said it had "major concerns" over the numbers and the Royal College of Nursing (RCN) said that European nurses' rights are "not clear enough".
The report, which examines activity on the register, states that overall, at the end of March 2018, there were 690,278 nurses and midwives on the register - 495 fewer than at the end of March 2017.
A survey of more than 3,000 British nurses identified retirement and staffing concerns as issues as two of the main reasons for nurses leaving the register.
However, there has been a leap in the number of nurses coming to Britain from outside of the EU - possibly due to recruitment programmes in places like India and the Philippines.
Last year it emerged the NHS was piloting an "earn, learn and return" programme which would bring international nurses to England to help plug staffing shortages.
Meanwhile, the new NMC report also shows that while the overall number of nurses on the register decreased, the number of midwives grew by 473 between March 2017 and March 2018.
"The number of people from the EU leaving our register remains a major concern, despite reassuring comments from senior members of government and nurse leaders," said NMC chief executive and registrar Jackie Smith.
"We asked people why they were leaving and for the small number of EU nurses and midwives who responded it's clear that Brexit is playing a part, while retirement and staffing levels are clear factors in the decision of UK nurses and midwives to leave the profession.
"The government has announced an increase in undergraduate training places and those responsible for workforce planning should continue to look at what can be done to better support the nursing profession at this difficult time."
Commenting on the report, Janet Davies, chief executive and general secretary of the Royal College of Nursing, said: "It feels that efforts to boost the number of nurses are being dragged down by a botched Brexit.
"Nurses returning home, or giving Britain a miss entirely, are doing so because their rights are not clear enough. Theresa May must use every opportunity to say they are welcome here and valued in health care.
"As the overall number of nursing staff falls again, it is patients who will worry the most. The Government knows patients can pay the highest price when staff shortages bite.
"Each country of the UK needs a funded and detailed plan to boost nurse numbers and ensure safe patient care."
A Department of Health and Social Care spokesman said: "It is encouraging to see the highest increase in the number of nurses and midwives from the UK joining the register for the first time in four years.
"We know staff have never worked harder, which is why we gave a pay rise to more than 1.2 million dedicated workers in the Agenda For Change pay deal, continue to work to improve retention and flexible working and have created thousands more training places to increase the number of nurses in the future."
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