The Royal College of Nursing (RCN) says Brexit poses an immediate risk to the provision of safe and effective care for patients in the United Kingdom.
The union has made the warning in a letter to leaders of the UK’s main political parties on behalf of its 435 000 members.
It also wants the parties to back a second referendum on any Brexit deal.
A government spokeswoman said plans are in place to ensure patients receive the same standards of care after Brexit.
The UK will leave the European Union on the 29th of March next year.
The chair of council at the RCN, Maria Trewern, warned that Brexit could exacerbate ongoing problems in the national health service, such as workforce sustainability.