"Extensive" planning is under way to prepare the health service for a no-deal Brexit scenario, the NHS England chief executive says.
Simon Stevens said immediate planning was taking place around the supply of medicines and equipment.
"Nobody's pretending this is a desirable situation, but if that's where we get to it will not have been unforeseen," he said.
Ministers say they do not want a "no-deal" scenario.
"I believe that is an option that can be very firmly avoided," Housing Secretary James Brokenshire told the BBC's Andrew Marr Show.
The UK is due to leave the EU in March 2019, and negotiations are taking place about what their future relationship will look like.
In recent days there have been several warnings that time is running out to reach agreement before the autumn, so that a deal can be ratified by politicians.
Theresa May's cabinet will gather at Chequers on Friday to try to agree a "clear direction" that the UK wants to follow in trade talks.
Last October Mr Stevens told MPs he had not been asked by the government to examine the potential impact of the UK leaving in March 2019 with no deal in place.
Asked about this scenario, he told Andrew Marr: "There is immediate planning which the health department, with other parts of government, are undertaking around securing medicine supply and equipment under different scenarios.
"That will obviously crystallise when it's clear later this autumn what the UK's position will be."
He also said every hospital had been asked to "reach out" to EU nationals working there with information about how to apply to stay in the UK.
There have been warnings about the UK leaving Euratom, which regulates Europe's nuclear industry, including the supply of medical isotopes which are essential for various types of cancer treatment.
Prime Minister Theresa May has called for the UK to have a "close association" with Euratom after Brexit.
She has also said the UK will seek "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.