NHS Nightingale hospitals in Manchester, Sunderland and Harrogate are being asked to get ready to take patients.
Government advisers say admissions are rising, with more elderly people needing urgent treatment for Covid.
More people are now in hospital with Covid than before restrictions were announced in March.
England’s deputy chief medical officer said a “marked pick-up” in cases would lead to more deaths.
Prof Jonathan Van-Tam said: “Already, with the cases that we know about, we have baked in additional hospital admissions and sadly we also have baked in additional deaths that are now consequent upon infections that have already happened.”
He warned that coronavirus was spreading from younger age groups into the over 60s who are more vulnerable.
Cases are rising across most of England, but the North West is the worst affected.
New local lockdown rules for England are due to be announced later, with the Liverpool City Region expected to face the tightest restrictions under a three-tier system according to risk.
Boris Johnson will set out the changes in the Commons on Monday afternoon, before speaking at a Downing Street press conference at 18:00 BST.
NHS England’s medical director Prof Stephen Powis cautioned that it would take “a number of weeks” before the benefit of any extra measures – such as shutting pubs – would be seen in bringing hospital admissions down.
He said: “In the over-65s – particularly the over-85s – we are seeing steep rises in the numbers of people being admitted to hospital so the claim that the elderly can somehow be fenced off from risk is wishful thinking.”