The NHS will need to start implementing some emergency preparations before Christmas if there is a no-deal Brexit, in order to be ready for leaving the European Union in March, MPs have been warned.
NHS England Chief Executive Simon Stevens said that some contingency plans would have to be enacted in December, with others needing to follow early in the new year.
His comments to the Health and Social Care Select Committee may place further pressure on MPs, who are due to vote on Theresa May's Brexit plan on December 11 and would have to seek a fresh agreement if they rejected it.
In the same hearing, Health and Social Care Secretary Matt Hancock said that pharmaceutical companies would have to shoulder any stockpiling burden if there was a no-deal Brexit, so that the NHS and individuals could receive medication and other supplies normally.
He also said that he did not "exactly" tell the Cabinet earlier this month that he could not guarantee that patients would not die if Britain left the EU without a deal.
In a lengthy hearing on Tuesday afternoon, Mr Steven was asked about contingency planning for Brexit.
He told MPs that NHS trusts have been told to report on their supply lines and contracts by the end of November to allow it to produce a comprehensive assessment in the first 10 days of December.
He added: "We will obviously review the situation comprehensively post-November 30 and I think frankly, this side of Christmas, alongside the plans, were we to be in a no-deal scenario then some of those plans would need to be enacted."
'Shouldn't use words like guarantee'
Mr Hancock was asked by Labour's Ben Bradshaw about reports that he told the Cabinet earlier this month that he "couldn't guarantee people won't die" because of a no-deal Brexit.
There have been fears about the supply of drugs and other medical supplies from EU nations in the event no deal is agreed before Britain leaves in March.
Mr Hancock replied: "We don't comment on leaks. That isn't exactly what I said and we have been very clear that if everybody does what they need to do then we can ensure continuity of supply."
Asked if this meant that he could guarantee that no-one would die if there was a no-deal Brexit, Mr Hancock added: "As my permanent secretary tells me, we shouldn't use words like guarantee.
"What we can say is that we are confident that if everybody does what they need to do then there will be the continuity."
Addressing the possibility of stockpiling in the event of a no-deal Brexit, he added: "We have been very clear to leaders in the NHS that the stockpiling is going to be done by pharmaceutical companies and we will support them and are supporting them in doing that.
"It does not need to be done above the normal levels of supplies by NHS trusts or by individuals.
"This is very important because if too many people try to stockpile then you end up with more demand for stockpiling, when what we need to do is stockpile within the pharmaceutical companies so they can fulfil their contracts in the normal way.
"So if you are in an NHS trust or pharmacy or in primary care or if you are an individual, then the supplies are available to you in the normal way and we are doing work to ensure that supplies will be uninterrupted."