Boris Johnson will lead a news conference today along with England’s chief medical officer, Sir Chris Whitty, and chief scientific adviser Sir Patrick Vallance.
The event at 5pm comes after multiple hospitals declared critical incidents due to staff shortages caused by coronavirus.
The Royal College of Nursing (RCN) told Health Secretary Sajid Javid the NHS could “ill afford” the current level of absences.
In a letter to Mr Javid it called for a “more cautious approach” to COVID restrictions in England “without further delay”.
The union also questioned the “confusing and concerning” differences in measures between the different UK nations.
It urged ministers to introduce “a more cautious approach for England without further delay” and said the move was supported by “high levels of staff absence due to COVID-19”, which doubled in the two weeks up to Boxing Day.
“The health and care system in England, already short tens of thousands of professionals, can ill afford the current losses,” it added.
Meanwhile, Mr Johnson warned it would be “absolute folly” to say the pandemic was over and the public must do everything they can to help relieve the pressure on the NHS by following plan B measures.
But the Prime Minister said Omicron was “plainly milder” than other variants and that the UK was in a better position than most other countries due to the “very, very high level of vaccination”.
Speaking at a vaccination centre in Aylesbury, Mr Johnson said: “My view is that all the evidence [shows] Omicron is much milder, considerably milder than previous variants.
“And we are much more boosted so our position is far, far better, but I think this is what I would say to everybody: looking at the pressures on the NHS in the next couple of weeks and maybe longer, looking at the numbers of people who are going into hospital, it would be absolute folly to say that this thing is all over now bar the shouting.
“We have got to remain cautious, we have got to remain with plan B, we have got to get boosted.”