NHS bosses are set to cancel urgent surgery across London in a move that could mean cancer patients waiting months for potentially lifesaving operations, the Observer can reveal.

NHS England chiefs are considering the drastic action because hospitals across the capital are becoming overwhelmed by people who are very sick with Covid-19.

The operations likely to be cancelled, known as “priority two” procedures, mainly involve surgery for cancer where specialists have judged that the patients need to be operated on within four weeks. Any delay could allow their tumour to grow, the disease to spread, or both, thus reducing their chances of survival.

Health service executives and cancer experts fear patients’ cancers may worsen, or even become inoperable, if surgery is postponed for an unknown length of time.

“These are operations that are curative if done within four weeks but if you wait longer they may not be so effective,” said one senior London NHS figure. “The impact of this on patients’ health depends on when they get rebooked. Delaying cancer surgery can lead to tumours growing or spreading – and worse outcomes.”

The situation has arisen because London hospitals are running out of intensive care beds and cannot perform types of cancer surgery that would require an ICU bed for the patient to recover in.

These operations are the second most pressing of the four categories of surgery the NHS undertakes. Priority four cases are elective procedures that can be deferred for longer than three months; priority three operations should be done within three months; and priority two within 28 days. The only types deemed more urgent are emergency operations, which need to be done within 24 or 72 hours.

Sir David Sloman, NHS regional director for London, said: “Londoners continue to receive urgent cancer care and surgery. Urgent cancer surgery is not being cancelled in London.”

The decision came as the UK recorded a further 57,725 cases of Covid-19, the fifth day running that the figure has topped 50,000. A further 445 deaths were also recorded yesterday.

Blood Cancer UK urged sufferers who were parents, in order to protect their own health, to seriously consider whether to send their children to school if they lived in an area where the infection rate was high. “No parent with blood cancer should feel under any pressure to send their children to school over the next few weeks if they think keeping them off is the right thing for their family,” the body said.

In another sign of the intense strain on the capital’s NHS, five hospitalson Saturday had to divert emergency patients to other hospitals because their A&E units were so overloaded.

Meanwhile, senior doctors and hospital bosses are warning that the NHS is at risk of becoming “maxed out”, with a widespread shutdown of normal care, because of the number of Covid cases needing hospital care.

They fear that the inter-household mixing that was allowed in some parts of the UK on Christmas Day will lead to a further rise in the already dramatic numbers of people becoming very ill. “When you factor in that any surge in infections due to Christmas mixing may only just have started to show, it’s clear the situation could get even worse over the next 10-21 days as the effects of Christmas and New Year become apparent,” said Dr Nick Scriven of the Society for Acute Medicine, which represents many hospital doctors.

“The NHS could well reach a ‘maxed out’ stage that will manifest itself as cancellation of all non-acute work, in terms of operations and clinics, causing further chaos to those waiting for treatment.” The crisis is so grave that the suspension of non-Covid care could last until Easter, he added.

Nicki Credland, chair of the British Association of Critical Care Nurses, said: “Some hospitals are running at up to 200% of their pre-Covid capacity, putting massive pressure on staff. We’ve not seen the effects of mixing on Christmas Day: there’s a two- to three-week timelag. The risk of overwhelming services is a very real one.”

Credland and NHS Providers, which represents hospital trusts in England, warned that the quality of care inpatients receive could suffer because of staff shortages and the strain on hospitals.

“There is significant staff sickness, which impacts on safe staffing levels. This will compromise patient care,” said Credland. Some trusts, she said, are already using one critical care nurse to care for three or even four very sick patients at a time, when the norm is 1:1.

Saffron Cordery, deputy chief executive of NHS Providers, said hospitals were supporting each other where they could, for example, by taking patients from busy neighbours: “There’s a real concern that things could get worse before they get better. Given the increasing demand, alongside high levels of staff absences caused by the virus, it will be harder to ensure the right quality of care for patients, many of whom are very sick.”

Dr Alison Pittard, the dean of the Faculty of Intensive Care Medicine, said: “I share the concern that the already dire situation facing the NHS will get even worse over the next few weeks, given the huge spike in infections and hospitalisations, and given that many hospitals are already close to being overwhelmed.

“Numbers [of people in hospital with Covid] are already higher than in April – and the relaxation of restrictions on Christmas Day, and people not following ‘hands, face, space’, will lead to increased transmissions, translating to hospital admissions in a few weeks.

“I don’t think the NHS will collapse. But it will be hard, and the NHS is likely to cancel some normal activity and perhaps use the Nightingale hospitals for less sick patients, to free up hospital capacity.”

Macmillan Cancer Support’s director of policy, Steven McIntosh, said: “Any cancer patient who sees delays to tests or treatment as a result of these extreme NHS pressures will be desperately anxious and scared. It is critical any changes to cancer care are carefully discussed with patients and are based on their individual needs, to ensure they access vital tests and treatment and are kept safe from the virus.”