The NHS could "go under" unless European Union staff are given reassurances about their future after Brexit, the head of the Royal College of Nursing has warned.
Janet Davies, chief executive of the nurses' union, said some EU workers were already leaving and hospitals would not be able to cope if significant numbers followed.
Theresa May has promised no EU citizen currently in the UK will be forced out after Brexit, but Ms Davies said the Government needed to give a clearer message to NHS staff and potential future recruits from the remaining 27 member states.
Significant differences still remain between Brussels and the UK over their proposals for citizens' rights after Brexit and Ms Davies said damage was already being caused by the uncertainty.
Ms Davies told the Sunday Times: "I understand there are lots of negotiations but if it is their aim to enable nurses to stay then they need to say that's what they are aiming to do.
"They need to give a clear message to those who are already here but also to those who want to come in after we leave.
"People will then be able to make a decision and if they decide that they don't want to stay then that is a big cliff-edge and my big fear is the NHS would go under.
"In some hospitals one in five members of staff are trained elsewhere in Europe and they just couldn't cope if suddenly everyone decided to leave."
"The risk is that people will start to leave and we are now starting to see that happen," she said. "We simply can't afford to lose any more nurses; we don't have enough as it is."
Figures from the Nursing and Midwifery Council (NMC) in July showed that the number of EU workers (who make up 5% of the register) leaving the professions had increased from 1,173 in 2012/2013 to 3,081 in 2016/2017.
"The risk is that people will start to leave and we are now starting to see that happen," Ms Davies said. "We simply can't afford to lose any more nurses; we don't have enough as it is."
The fall in the value of sterling had also affected the NHS's ability to recruit and retain EU staff, she said, and also pointed to the Brexit vote's impact on morale.
"Some of them really feel it especially when they have been asked by some patients 'why are you still here?'"
Ms Davies wants European Economic Area NHS workers to be granted permanent residence without the need for "expensive and lengthy administrative burdens on employers and individuals".
She also called for sufficient notice of any "cut-off date" and the move to a post-Brexit immigration system as well as assurances that hospitals would continue to be able to recruit from overseas during a transitional period after the UK's withdrawal from the EU in March 2019.
"There are clear messages the government can give out but they are absent at the moment," she said. "One is that you can stay in the country and the other is that we are going to pay you properly."
The RCN is holding a summer of protests and is threatening a ballot on industrial action later this year if the cap on public sector pay rises is not lifted.
In an ultimatum to Chancellor Philip Hammond, Ms Davies called for the cap to be lifted in the Budget this autumn.
She added: "We have been working as hard as we can to make the Government understand that if you want a healthy health service we need a healthy salary for our nurses.
"If the budget does ot show the government's clear intent to remove that pay cap then industrial action by nurses is on the table."
A Department of Health spokesman said: "Nurses are the bedrock of the NHS, and we understand the need to give certainty to valued staff from the EU. That is why we have made clear that the future of EU nationals working in our health and care system should be a priority in Brexit negotiations.
"We have invested in the frontline and since May 2010 there are almost 32,400 more professionally qualified clinical staff on our wards - including over 12,500 more nurses."