Widespread nursing shortages across the NHS could lead to staff burnout and risk patient safety this winter, the Royal College of Nursing has warned.
The nursing union said a combination of staff absence due to the pandemic, and around 40,000 registered nursing vacancies in England was putting too much strain on the remaining workforce.
The government says more than 13,000 nurses have been recruited this year.
It has committed to 50,000 more nurses by 2025.
It also hopes England's four-week lockdown will ease pressure on the NHS.
The RCN has expressed concern that staff shortages are affecting every area of nursing, from critical care and cancer services to community nursing, which provides care to people in their own homes.
The union said it was worried the extra responsibility and pressure placed on senior nurses could lead to staff "burnout", as hospitals struggle to clear the backlog of cancelled operations from the first wave of coronavirus and cope with rising numbers of new Covid patients, as well as the annual pressures that winter typically brings.
The boss of NHS England, Sir Simon Stevens, told a Downing Street press conference on Thursday that flu and other winter conditions usually brought in 3,000 additional patients to hospitals, but there were already more than 10,000 coronavirus patients in England before winter pressures have even arrived.
The latest government figures show there were more than 12,000 Covid patients in hospital in the UK on 3 November.
Sir Simon said some staff had been trained and redeployed to help bolster care for Covid patients in ICU, but the RCN said it may not be enough to cope with a surge in demand and has called on the government to think carefully about which services can be safely maintained, without spreading staff "too thinly".
Mike Adams, RCN England Director, said: "The NHS is now at its highest level of preparedness as it faces the prospect of an extremely challenging winter.
"We already know that frontline nurses – in hospitals, communities and care homes – are under huge strain. Anecdotally we're hearing that in some hospitals they [nurses] are becoming increasingly thinly spread on the ground, as staff become unwell or have to isolate, at the same time as demand on services continues to increase.
"The government says nurses have been given extra training to provide more critical care staff to treat Covid-19 patients, but there simply aren't enough to go around. There are around 40,000 registered nursing vacancies across the NHS in England alone.
"It is essential that learning is applied to planning for this winter, including what services can be delivered safely with the workforce available."