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 a new three-tier system of lockdown rules for England is due to be announced.
Boris Johnson will set out the changes in the Commons on Monday afternoon, before speaking at a Downing Street press conference later.
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Three-tier lockdown system to be unveiled in England
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The Liverpool City Region is 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.
Liverpool recorded 600 cases per 100,000 people in the week ending 6 October. The average for England was 74.
