Hospitals across the UK are beginning to cancel non-urgent surgery and specialised clinics as coronavirus cases surge again.
All non-urgent surgery, outpatient clinics and specialist nurse services are already on hold in parts of South Wales as staff are redeployed to cope with COVID-19 cases.
Barts Health NHS Trust, which serves around 2.5 million people in east London, said it had moved to the "high pressure" phase of its winter escalation plan which included deferring some routine procedures.
And Mid and South Essex NHS Foundation Trust said it was also reviewing non-urgent operations, which would lead to postponements.
A Barts Health spokesman said: "We are treating high numbers of patients with COVID-19, and in line with our winter escalation plan we have moved into a 'high pressure' phase and are taking steps to keep our patients safe.
"These include deferring some routine procedures over the coming days so we can redeploy staff and increase the number of critical care and general beds available."
The trust, which operates across four major hospital sites The Royal London, St Bartholomew's, Whipps Cross and Newham, said the plan will not affect cancer patients and that people will be contacted directly if their elective procedures need to be postponed.
Mid and South Essex NHS Foundation Trust deputy chief executive Tom Abell said the trust was dealing with a rising number of coronavirus patients and urged people to follow the rules.
He said: "The hospital is open and patients should attend planned appointments and procedures as normal unless we contact them to say otherwise.
"The number of COVID-19 patients we're caring for is increasing and it's vital that our patients and the local community follow the national guidance to help keep people safe."
The total number of COVID-19 patients in hospital in England stood at 15,465 on Wednesday, up from 13,467 a week earlier.
During the first wave of the virus, this number peaked at 18,974 on 12 April.
In Wales, a joint statement representing the three areas of Bridgend, Rhondda Cynon Taf, and Merthyr Tydfil said that in the past week, the equivalent of almost four people acquired the virus every 10 minutes.
"The transmission rates in our council areas are in the highest 20 areas in the whole of the UK," the statement said.
Urgent cancer services will continue and other urgent patients will be seen, a joint statement from Cwm Taf Morgannwg University Health Board and Bridgend, Rhondda Cynon Taf and Merthyr Tydfil County Borough Councils said, adding the region's vaccination programme will continue.
"This decision has not been taken lightly as we know it will mean people in our communities will not be able to access some local NHS services and many patients will have to wait longer for outpatient appointments and operations," they said.
"However, the rates of transmission and the associated rise in hospital admissions mean that there is no choice if we are to keep essential and emergency services running."
Services that have been stood down in Bridgend, Rhondda Cynon Taf, and Merthyr Tydfil include:
All non-urgent outpatient clinics - this includes face to face and virtual
All non-urgent planned surgery
All specialist nurse clinics
All non-urgent diagnostic services (eg radiology/endoscopy)
The move comes after the Welsh government said it was limiting mixing at Christmas to two households, rather than the previously announced three.
The country will also go back into lockdown for three weeks from 28 December, closing all non-essential retail, hospitality venues and hairdressers.