The group representing hospitals and ambulance services in England has warned of a lack of "contingency planning" to deal with the impact of a no-deal Brexit on the health service.
In a leaked email to NHS England boss Simon Stevens, NHS Providers says leaving the EU without agreement would immediately be a real risk to services.
The group warns it would make it harder to stop the spread of diseases.
NHS England said preparing for every potential Brexit outcome is a priority.
Ministers are to reveal more details of their contingency planning for a no-deal Brexit later this week.
Brexit Secretary Dominic Raab, who will travel to Brussels for the latest round of talks with his EU counterpart Michel Barnier on Tuesday, has said a deal is still the most likely outcome.
But he has said preparations for the failure to reach an agreement this autumn, or Parliament rejecting any deal, is the responsible course of action.
The UK is set to leave the EU on 29 March 2019 and the two sides are currently negotiating the terms of its exit and its future relations in a whole range of issues.
With seven months to go, ministers have said they are stepping up contingency preparations for a no-deal Brexit, while insisting that it is not their chosen outcome.
'Stockpiles and shortages'
But NHS Providers – which represents acute, ambulance, community and mental health services within the health service – has expressed concern about what it says is a lack of engagement with ministers in the email, seen by the BBC.
It has called for NHS England and NHS Improvement – which oversees NHS trusts and providers – to convene a group of trust leaders as a matter of urgency.
In an email sent to NHS chief executive Simon Stevens, also copied in to Mr Raab and Health Secretary Matt Hancock, it calls for a co-ordinated response to confront the challenges that would be presented by a no deal.
Chief executive of NHS Providers Chris Hopson writes that there has been "no formal communication" to trusts from either NHS England or NHS Improvement on this issue.
Without national planning and coordination "there could be both stockpiles and shortages of medicines and medical devices", Mr Hopson says.
He adds that "disease control coordination could also suffer".
'An insight into NHS thinking'
Analysis by Chris Mason, BBC political correspondent
This is a private letter from one part of the NHS to another.
So, unlike the noisy public rows at Westminster about Brexit, it is an insight into how managers in the health service are, among themselves, fretting about the practicalities of leaving the European Union without a deal.
When I rang the Department of Health for its reaction to this, a spokeswoman refused to comment.
The government will point though to two things it is up to: the ongoing attempts to get a Brexit deal and the imminent publication of its contingency plans for a no deal scenario, including for the health service.
But it is now increasingly clear that while attempts to reach a deal rumble on, concerns about a failure to do so are growing louder.
Two MPs who want a referendum on the final Brexit deal have criticised the government's approach.
Conservative MP Dr Sarah Wollaston, chair of the health and social care committee, said: "For the government to proceed without setting out the risks and benefits of the terms of Brexit…would be like a surgeon relying on consent to surgery obtained two years previously – without their patient knowing whether this would involve a few toes or their whole leg."
Labour MP Ben Bradshaw said: "No-one voted for this shambles and they certainly didn't vote to damage our NHS."
An NHS England spokeswoman said the health service was working with government, planning for different post-Brexit scenarios.
"We will be working with our colleagues and partners across the NHS to ensure plans are well progressed, and will provide the NHS with the support it need," she said.
Last month, Mr Stevens said "extensive" planning was under way to prepare the health service for a no-deal scenario, to ensure the continued supply of medicines and equipment.
While this would not be "desirable", he said that – if such a situation arose – the effects would not be "unforeseen".
A series of technical notices – including advice for businesses, citizens and public bodies about a so-called "hard Brexit" – will be made public over the next month or so.
Downing Street has described the advice due on Thursday as "sensible, proportionate, and part of a common sense approach to ensure stability, whatever the outcome of talks".
On the same day, Mr Raab will make a speech in Westminster to outline the government's plans for the possibility of leaving the EU without a deal.
The UK is seeking "associate membership" of the European Medicines Agency, which evaluates and supervises medicines and helps national authorities authorise the sale of drugs across the EU's single market.
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