There are fears the NHS could suffer a shortage of nurses after Brexit, according to a report by the Health Service Journal (HSJ).
A leaked document reportedly seen by the HSJ cites the "worst case scenario" of a shortage of 42,000 nurses.
This would only happen if all new international staff were to stop coming to the UK in 2019.
The Department of Health forecasts a shortage of nurses of between 26,000 and 42,000 by 2025-2026, the document reportedly said.
The journal quoted information they had seen as saying: "The analysis indicates that there is a severe risk of undersupply if immigration rules change and international inflows stop."
Experts have branded such an outcome "unsafe", adding that decisions would have to be made over what treatments could be provided.
The numbers were presented to ministers in March, the journal said.
Janet Davies, chief executive of the Royal College of Nursing, told the journal such a fall in healthcare staff "would be unsafe".
She said: "Decisions would have to be made about what would we be able to provide.
"You wouldn't want to keep running an unsafe service that was so short of nurses."
Of the estimates, Ms Davies said: "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 Secretary of State has always made clear that EU nationals who work in the NHS make a valuable contribution and this will be taken into account during Brexit negotiations."
In response shadow health secretary, Jonathan Ashworth called for a guarantee on EU workers' rights.
Mr Ashworth said: "When Parliament returns, the Secretary of State must update the House on his response to this independent civil service analysis and finally outline his plans for the NHS staffing and Brexit.
"It's time for an 'NHS guarantee' for these workers ensuring their rights - offering these workers who care for our sick and elderly the certainty that they deserve."