RCN chief executive Pat Cullen said a “very, very depleted workforce” was facing a worse Christmas than last year due to the number of staff forced into isolation.
Ministers would find it easier to make decisions on how to defend the health service if they “walk in the shoes of any nurse” for a day, Ms Cullen said on Friday.
Healthcare staff are often working 14-hour shifts to make up for the shortages, she said.
“We need to listen to the wonderful scientific experts that we have throughout the country,” Ms Cullen said.
“We listened to them yesterday evening and many evenings on TV and what (they are) saying that something needs to happen in terms of perhaps a circuit breaker, and that if we leave it much longer unfortunately our nurses fear it will be a little bit too late for the health service.”
Some 3,874 NHS staff at acute hospital trusts in London were absent for Covid reasons on December 19, more than double the number a week earlier (1,540) and more than three times the number at the start of the month (1,174), according to new figures from NHS England.
The total includes staff who were ill with Covid-19 or who were having to self-isolate.
Across England as a whole, 18,829 NHS staff at acute trusts were absent due to Covid-19 reasons on December 19, up 54% from 12,240 a week earlier and up 51% from 12,508 at the start of the month.
Ms Cullen said those figures were only likely to increase over the festive period.
“The despair that nurses are facing and the fear and the sheer struggle they’re facing every day will continue over the Christmas period, because this Christmas, of course, will not be normal. It will be similar to last year, if not worse, is what they’re telling us,” she added.
Her concerns about the service becoming overwhelmed were echoed by Chris Hopson, head of NHS Providers, who said the NHS is under “huge amounts of pressure”.
“We’ve got growing staff absences because as soon as Omicron starts circulating in the community, surprise, surprise, you’ll find that NHS staff get the infection and therefore have to take time off,” Mr Hopson told Sky News.
There were 2,097 people in hospital in London with Covid-19 as of 8am on December 23, according to new figures from NHS England, the highest number since February 27.
During the second Covid wave, the number peaked at 7,917 on January 18.
Across England, 7,114 patients were in hospital with Covid-19 on December 23, the highest number since November 4 and up 11% week on week.