Britain’s National Health Service “is now on a war footing,” one of its top medical officials warned on Thursday, saying its hospitals would erect field wards to help absorb the impact of a record surge in coronavirus cases that has already produced a rise in hospitalizations nationally.

Cases in Britain reached new highs this week, driven by the highly transmissible Omicron variant, with more than 183,000 reported across the country on Wednesday — twice the highest daily count recorded in previous waves. And public health experts have said it is likely to be days before the full impact of socializing over the Christmas holidays is reflected.

“We do not yet know exactly how many of those who catch the virus will need hospital treatment,” Prof. Stephen Powis, the N.H.S. medical director for England, said in a statement. “But given the number of infections we cannot wait to find out before we act.”