Hospitals across England have been told not to carry out planned operations over Christmas as they deal with increasing pressure on the system.
In a letter to NHS trusts, chief executive of NHS Improvement, Jim Mackey, said the focus needed to be on emergency patients at a "critical" time.
It comes amid warnings over increasing pressure as winter bites, with hospitals becoming full and A&E departments struggling to hit the target to deal with patients within four hours.
Figures out today show that 37 NHS trusts reported they had been at level three on the operational pressures scale on at least one day in the week ending December 11. This level means the "local health and social care system is experiencing major pressures".
Three trusts were at the highest level four during the week, meaning "pressure in the local health and social care system continues to escalate leaving organisations unable to deliver comprehensive care".
This level also means there is "increased potential for patient care and safety to be compromised".
As a result of outbreaks of norovirus, there have also been more bed closures than at the same point last year. There was an average of 860 beds per day closed across the NHS, up from 211 at the same point last year.
In the new letter, Mr Mackey says trusts should "cease" most planned operations and activity over Christmas, also known as elective care, to ensure enough beds are available.
But it said NHS Improvement expects patient cancellations to be kept to a minimum.
The letter, seen by the Health Service Journal (HSJ), said: "In preparing for managing winter pressures, it is recommended that all providers 'pace' their elective work by introducing elective breaks where trusts cease most in-patient elective activity and focus on treating emergency activity and non-admitted patients."
It added: "Given the level of risk facing the system, it is clear that having sufficient bed capacity going into Christmas is critical, and we know most organisations will already have this in hand as part of local planning arrangements."
The letter says cutting bed occupancy rates to 85% - the recommended amount to keep infection rates low and create "slack" in the system in case there is an outbreak of something like flu - will better enable trusts to deal with unplanned patient care and emergencies.
The rate of 85% should be managed between December 19 and January 16, the letter said. Figures out today show beds are currently running at just over 95%.
The letter said day cases should continue as normal, and planned activity should be re-started "on a phased basis".
NHS Digital figures show that in 2015/16, there were approx 7,447,172 planned operations. That is an average of 620,598 per month.
Shadow health secretary Jonathan Ashworth said: "This is yet another indication of how the NHS is struggling to the cope with the increased pressures during winter.
"The Tory failure to provide the NHS with the funding it needs means that hospitals are having to close operating theatres over Christmas just to get through the winter.
"This short-term fix is only going to store up problems and leave more and more people stuck on waiting lists. The Tories need to take urgent action now to provide the NHS with the funding it needs."
NHS Providers chief executive, Chris Hopson, told HSJ: "This guidance does go further than before – for example in specifying a target bed occupancy level as the holiday period starts – but it's in line with direction of travel over the last few years, prioritising emergency over elective work for the holiday period.
"Many trust chief executives tell us they were already planning along these lines, but this is a helpful prompt to ensure the work is completed to time and quality."
A spokesman for NHS Improvement said: "NHS providers will be doing all they can to make sure their patients are able to receive quality care during the busy Christmas period. A reduction of elective hospital activity in the run-up to Christmas is standard practice, and well-rehearsed by NHS providers.
"Many hospital trusts also routinely wind down elective activities in the run-up to the Christmas and New Year period, as patients do not wish to be in hospital over the festive period and those who are medically fit for discharge want to return home. This also frees additional capacity.
"Cancellations should be kept to an absolute minimum and, in trusts where elective work does need restricting, then the decisions should always be taken using appropriate clinical reviews to ensure patient needs are met."