The NHS faces a shortage of up to 42,000 nurses in the six years after Brexit, according to a leaked report.
The Department of Health document envisages a "worst case scenario" if EU and non-EU nurses stop coming to the UK once it leaves the EU.
According to the projection, by 2025/26, the NHS would be short of between 26,000 and 42,000 staff and that there is a "severe risk of under-supply if immigration rules change and international inflows stop".
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Sky News says ministers were reportedly shown the forecast in March.
Janet Davies, chief executive of the Royal College of Nursing, said it would be "unsafe" to continue running a service that was so short of nurses and that "decision would have to be made" about what the NHS could provide in the wake of large-scale staff reductions.
She added: "We warned of this years ago and we were told we were scaremongering.
"This is a large problem that is coming at us quickly and we need to move fast. What we are not seeing is any real action."
A spokesperson for the Department of Health said: "As you would expect, the department and others are focused on workforce planning to ensure the NHS has the staff it needs to continue to provide good care."
The news comes as Pulse Today magazine reports that record numbers of GP practices are closing, forcing as many as 265,000 patients to change doctors surgeries in 2016.
A Freedom of Information request revealed 57 surgeries had shut last year, with another 34 closing because of practice mergers.
The Royal College of GPs said doctors could no longer cope with growing patient demand without more funding, reports the BBC.