The Aberdeen Royal Infirmary (ARI) is also at this level, with bed capacity “limited”.
It comes as soldiers began working to help support NHS Lanarkshire and NHS Borders this week, while two further health boards – Grampian and Ayrshire and Arran – have also asked for assistance.
NHS Lanarkshire’s three general hospitals – Hairmyres, Monklands and Wishaw – are at “critical occupancy levels”, said Laura Ace, deputy chief executive of the health board.
Non-urgent surgery has been suspended since August and this has now been further extended, with some cancer procedures also delayed.
Ms Ace said: “The current situation is unprecedented and marks a different level of risk for NHS Lanarkshire as a whole and moves our current status to the highest level of risk.”
NHS Lanarkshire’s Strategic Command Group has moved the health board from “red” status to “black”, the highest level.
Ms Ace said the problems were down to “relentless pressures, bed shortages and staff shortages due to sickness, stress and self-isolation”.
These show “no signs of easing”, she said.
“The safety of our patients and staff is our top priority and we are working through short and medium-term actions to increase staffing and also improve the flow of patients out of hospital,” she said.
NHS Grampian said acute hospitals in the area were “extremely busy”, and the Aberdeen Royal Infirmary has reached the highest stage of pressure.
Some non-urgent surgery has been cancelled, but cancer procedures continue.
"While we do not recognise or use the term 'Code Black', ARI is at the highest stage of pressure and bed capacity is limited,” a spokesperson said.
"We continue to work hard to manage patient flow through all hospitals, supporting the discharge of those individuals who are fit to go home, or to other facilities.
"We are maintaining all emergency surgery and working to maintain, wherever possible, all time critical planned procedures. This includes cancer procedures.
"Unfortunately, we continue to see postponement of elective procedures, typically due to not having suitable post-surgery beds available. Postponement is always a last resort and procedures will be rearranged as quickly as possible."
Scottish Labour health spokesperson Jackie Baillie said the level of pressure in NHS Lanarkshire “puts it beyond doubt that we are in the midst of a full-blown NHS crisis”.
She said: “It is a damning indictment of the SNP’s recovery plan that risk levels in Lanarkshire are now higher than they were when Covid was at its peak.
“Staff are working tirelessly to do right by patients, but services are simply stretched past breaking point.
“It is a national scandal that cancer operations are being cancelled when we are already playing catch-up and it is frankly terrifying that we have reached this point before winter has hit.
“Lives are on the line because of the SNP’s negligence.
“They need to wake up to the scale of the emergency they’ve created and come up with a recovery plan that is fit for purpose.”
Liberal Democrat leader Alex Cole-Hamilton urged the government to take “immediate action” to support NHS Lanarkshire.
“Cancelling planned, potentially life-saving operations, including cancer operations, is shocking,” he said.
“It is crystal clear that our NHS needs a recovery plan not just from the pandemic, but from 14 years of SNP neglect. After over 18 months of pandemic and as winter approaches, the government should have had a real plan in motion.
“Scottish Liberal Democrats will continue to press for fresh resources to alleviate the burden on staff and ensure that long-abandoned targets for cancer operation are finally met.”
Soldiers began working with both NHS Lanarkshire and NHS Borders this week.
A total of 86 personnel have been deployed, initially until November 10.
A spokesperson for NHS Borders said the Borders General Hospital was at “red” level, one lower than black.
NHS Grampian and NHS Ayrshire and Arran have also issued requests for military assistance, which are still being considered by the Scottish Government.
It is understood that no hospitals in NHS Ayrshire and Arran are currently at maximum capacity.
"NHS Ayrshire & Arran continues to experience significant pressures across our health and care services,” said interim chief executive Professor Hazel Borland.
"Covid-19 admissions, staff absence and rising emergency attendances, has had a significant impact on how we provide health and care services in Ayrshire and Arran.
"This pressure is being felt right across acute services, and primary and community care, with unprecedented demand for services compounded by staff absences.
"NHS Scotland remains under emergency planning conditions, and our emergency management team remains entirely focused on taking action to ease the pressures on our health and care system and support patient and staff safety.”
A Scottish Government spokesperson said the situation in NHS Lanarkshire was “unprecedented”.
“The Scottish Government continues to actively monitor the situation and is taking direct action to support NHS Lanarkshire, this includes having secured the provision of temporary military assistance at each of Lanarkshire’s acute sites,” they said.
“The health secretary has made it absolutely clear that the NHS is facing its most difficult challenge ever in its 73-year history.
"We know that the pandemic is not over, and that COVID-19 and other pressures will continue to impact the NHS for some time. He and officials remain in daily contact with health boards to ensure the safety of patients and staff at this very difficult time."