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.
It comes as new local lockdown rules for England are due to be announced later to get infection rates down.
England’s deputy chief medical officer said the “marked pick-up” in cases that the country is seeing would lead to more deaths.
Prof Jonathan Van-Tam said the rise in coronavirus cases was now being seen “nationwide” and was not solely a problem for northern England.
He warned that coronavirus was spreading from younger age groups into the over 60s who are more vulnerable.
Hospitals have not yet reached capacity, but the NHS may have to use some of the temporary critical care Nightingale hospitals if demand continues to rise, say the advisors.
The Liverpool City Region expected to face the tightest restrictions under a new three-tier system which will classify regions as being on “medium”, “high” or “very high” alert.
Boris Johnson will set out the changes in the Commons on Monday afternoon, before speaking at a Downing Street press conference later.
Liverpool recorded 600 cases per 100,000 people in the week ending 6 October. The average for England was 74.
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.
“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,” he said.
On Sunday, 12,872 people in the UK were reported to have tested positive for coronavirus – some 2,294 fewer than on Saturday.
There were a further 65 deaths – down from 81 on Saturday.