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, 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, including one in South Tyneside, there have also been more bed closures than at the same point last year - an average of 860 per day, 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, to ensure enough beds are available.
The letter, seen by the Health Service Journal, said it is recommended that trusts "cease most in-patient elective activity and focus on treating emergency activity and non-admitted patients".
It adds: "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%.