The Covid-19 crisis will see the number of people waiting for NHS treatment double to 10 million by the end of the year, health bosses said.
The NHS Confederation said the challenges include backlog of cases, maintaining social distance and staffing.
The organization, which represents health and care leaders, said emergency funding and longer-term spending were needed.
The Department of Health has said it will continue to provide resources, funding and support for the needs of the NHS.
Guidelines have been issued on how the NHS “should start restoring services safely,” added a spokesperson.
He intervenes as Prime Minister Boris Johnson is to announce new plans to ease lock-in restrictions in England later, with burglary zoos and cinemas among the businesses that should be allowed to reopen from Monday.
Johnson will head the daily Downing Street briefing, where he is also likely to question government abolition plans to get all primary school children in England back to school before the summer holidays.
NHS Confederation projections show that the NHS waiting list is expected to drop from about 4.2 million currently to around 10 million by Christmas.
This assumes that the health service returns to full capacity within the next 12 months.
In a new report, the NHS Confederation said health services were operating at approximately 60% reduced capacity due to infection control measures.
The organization – which covers organizations that order and deliver health services in England, Wales and Northern Ireland – said it was facing a “difficult battle” as it attempted to restart the cancer, stroke and heart care, while continuing to manage thousands of patients and recover Covid. -19 patients.
Health leaders have urged the government to prepare the public not to expect the same level of service for several months.
In the past fortnight, cardiology services have restarted in England, but the British Heart Foundation warned on Friday of a delay in the procedure, as about 28,000 cardiac procedures had been delayed since the coronavirus epidemic.
Cancer services are also starting to reopen.
Cancer Research estimates that around 2.4 million people in the UK are waiting for screening, treatment or testing with the potential for 23,000 cancers to go undiagnosed during the lockout.
The already long waiting lists “would certainly increase significantly,” the body said, and coronavirus patients would continue to need recovery care, including respiratory and psychological care.
He added that care was provided by “exhausted and traumatized staff” and that health officials must remain ready for a second wave.
In its study and cover letter to the Prime Minister, the NHS Confederation warned that it would not be possible to simply “activate” NHS services immediately.
He also asked for new assurances about the effectiveness of the testing and tracing program and for new guarantees on personal protective equipment (PPE).
And the report says a review of the pandemic’s impact on the NHS and social care staff is needed.
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The Royal College of Nursing has warned that it will be difficult for “exhausted” nurses in services, nursing homes or understaffed clinics to restart services.
“The legacy of this pandemic has not yet come – professionals are still focused on the here and now,” said a spokesperson.
“As services begin to return, the government must continue to invest in the workforce so that an exhausted profession … is properly supported. ”
Secretary of State for Occupational Health Jonathan Ashworth called on ministers to “heed these warnings.”
“It is inevitable that the Covid-19 pandemic will have an impact on our health services in the months to come, but it is vital that ministers begin to address this backlog of delayed treatment and growing clinical needs,” did he declare.
NHS Confederation Executive Director Niall Dickson said health service leaders understood “the need to ease the lockdown and get the country back to work”.
“Part of that will involve restarting diagnostic testing, routine operations, outpatient appointments and other care, but we need to do it safely,” he said.
He said there was “a real determination to take on this challenge”, but added “that he will need additional funds and capacity, especially in rehabilitation and recovery services in the community”.
The confederation has asked for an extension of the agreement with the private sector, in order to provide beds, equipment and personnel to the NHS, until next March.
A spokesperson for NHS England previously told the BBC that the NHS was “bringing other services safely” and “dramatically increasing rehabilitative care for all those suffering from the effects of the virus.”
NHS England released a plan in May explaining how to increase routine operations and treatments.