Vaccine shortages are likely to cause problems for "several months", England's chief medical officer has warned, amid fears that emergency healthcare staff at Covid "battle stations" are at risk of burnout.

Professor Chris Whitty said the UK needs to urgently maximise the number of people who are vaccinated, as he defended a shift to prioritise first doses for as many at-risk people as possible.

But he said a lack of global supplies will likely hamper efforts to protect the nation in the first part of 2021.

A letter signed by Professor Whitty and the chief medical officers for Scotland, Wales and Northern Ireland, said: "We have to ensure that we maximise the number of eligible people who receive the vaccine.

"Currently the main barrier to this is vaccine availability, a global issue, and this will remain the case for several months and, importantly, through the critical winter period.

"Vaccine shortage is a reality that cannot be wished away."

The makers of the Pfizer/ BioNTech vaccine said they are working flat out to boost production of their Covid-19 vaccine, but they warned there will be gaps in supply until other vaccines are rolled out.

"At the moment it doesn't look good - a hole is appearing because there's a lack of other approved vaccines and we have to fill the gap with our own vaccine," BioNTech CEO Ugur Sahin told news weekly Spiegel.

Last week Jonathan Van-Tam, deputy chief medical officer, said a shortage of "fill and finish" materials needed to produce and package vaccines could also slow down the national rollout.

At a Downing Street news conference Professor Van-Tam said: "Many of you know already that it's not just about vaccine manufacture. It's about fill and finish, which is a critically short resource across the globe."

The warnings came as the UK faces a rising number of Covid patients in hospitals across the nation.

Adrian Boyle, vice president of the Royal College of Emergency Medicine, said many medical staff feel "tired, frustrated and fed-up".

Speaking to BBC breakfast, he voiced concerns over a possible burnout: "What is it going to be like over the next couple of months? I don't know, I am worried."

"We are very much at battle stations.

"There will be short-term surges of morale but people are tired, frustrated and fed-up, as everybody is, whether they work in hospital or not."

His comments came as the Royal College of Nursing's England director, Mike Adams, said that staff leave was being cancelled to deal with the surge in demand.

Meanwhile, Nightingale hospitals across England are being readied for use to take pressure of hospitals.

But Mr Adams said the expectation of a mass rollout of staff for the Nightingale hospitals was "misplaced".

He added: "If we are having to cancel leave to staff these areas, the obvious question is where will the staff come from to open the Nightingales?

"I have real concerns that the expectation that this mass rollout in capacity can happen is misplaced because there aren't the staff to do it."

On Thursday, the UK's chief medical officers backed a change in guidance which says booster jabs should be given up to 12 weeks after an initial dose to maximise the number of people being vaccinated.

But the announcement on Wednesday prompted Pfizer to issue a warning over what it called "alternative dosing".

A spokesperson said: "There are no data to demonstrate that protection after the first dose is sustained after 21 days."

However, the UK's chief medical officers said: "In terms of protecting priority groups, a model where we can vaccinate twice the number of people in the next 2-3 months is obviously much more preferable in public health terms than one where we vaccinate half the number but with only slightly greater protection."

Pressure is mounting on the government to control the spread of the Covid-19, which has continued to reach new heights.

The UK recorded a record high of 55,892 coronavirus cases in 24 hours on Thursday, as well as a further 964 deaths within 28 days of a positive test.

Meanwhile, just under half of major hospitals in England currently have more Covid-19 patients than during the peak of the first wave last year.

But GPs have said changes to the way vaccinations are given are "grossly unfair" for elderly patients with imminent appointments for a vaccine booster.

Dr Richard Vautrey, chairman of the BMA GP committee, said: "This group of very elderly patients is at the highest risk of death if they contract Covid-19, which is why GPs are so concerned for them.

"It is grossly and patently unfair to tens of thousands of our most at-risk patients to now try to reschedule their appointments.

"Local leaders are telling us that is unprofessional and impractical to amend the appointments for thousands of frail elderly patients, particularly those booked and who have already made arrangements to have their second vaccination in the next two weeks."

Latest figures show 944,539 people have been given the first dose of the coronavirus vaccine in the UK.

The Oxford/AstraZeneca vaccine will be rolled out alongside the Pfizer jab from Monday. The UK has 100 million doses on order.