Spending cuts have created a shortage of 20,000 NHS nurses, the Government has been warned, as fears grow that hospital wards may struggle to cope as winter approaches.

Freedom of Information requests submitted by the Royal College of Nursing (RCN) to dozens of NHS hospitals in England have exposed a “hidden workforce crisis” that has been missed by government statistics.

While official figures say that just 3,859 full-time nurse, midwife and health visitor posts have been lost since the Coalition came to power in May 2010, the RCN said that thousands more nursing vacancies have been created because hospitals have not been replacing staff that have retired or moved on due to reduced budgets.

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Staffing shortages have been highlighted in a number of reports into NHS care. Robert Francis drew attention to understaffed wards at the Mid Staffordshire NHS Trust in his report into one of the worst care scandals in the health service’s history.

Howard Catton, the RCN’s head of policy, said that Government figures had not been “fully reflecting the shortages [that nurses] are experiencing at ward level”.

The report came as Downing Street confirmed that the Prime Minister is personally overseeing the NHS’s response to what A&E doctors have warned could be “our worst winter yet”. Many trusts missed their A&E targets last winter and there are fears that amid rising demand and reduced resources, the system may struggle to cope with expected spikes in admissions.

Andy Burnham, Labour’s shadow Health Secretary, said the RCN’s report raised “a worrying prospect that the NHS is going into winter with 20,000 fewer nurses than it should have”.

The RCN found that, out of 61 trusts that provided data, the average “vacancy rate” for nursing posts was 6 per cent – up from just 2.5 per cent in 2010. Across the NHS, such shortages would amount to nearly 20,000 fewer nurses than needed to provide the best possible care to patients.

Vacancy rates are determined by the difference between the number of nurses that hospital managers estimate they need and the number that they actually have. The Government used to collect the data, but temporarily stopped doing so in 2011 as part of a drive to save money on bureaucracy in the NHS. At some hospitals, the vacancy rate for nursing posts is as high as 16 per cent.

A simultaneous 15 per cent cut to the number of nursing student places since 2010/11 has led to many NHS trusts seeking to recruit from overseas, the RCN said.

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The Department of Health said it would soon resume collecting vacancy data and said the health watchdog the Care Quality Commission would take action against any hospitals found to be operating without enough staff.

However, Mr Catton said that, by failing to monitor vacancy rates, the Government had missed warnings on staffing levels. He said: “We are concerned. We think it’s an important indicator of the workforce. It’s akin to the petrol gauge in your car. It tells you what you’ve actually got but also how much you need.”

Q&A: Why A&E departments are under pressure

Q | What are A&E wards supposed to be for?

A | The Royal College of Emergency Medicine warned last week that A&E must not be interpreted as “anything and everything”. It means accident and emergency – the NHS recommends that, generally, patients should visit for “life-threatening emergencies”.

Q | Why are they becoming overcrowded?

A | Demand has increased and beds in other hospital wards are in short supply. Opinions differ as to why demand is up. Britain has an ageing population, and older people are more likely to need emergency care. There has also been a shift in patients with more minor conditions going to A&E when before they would have seen their GP. Jeremy Hunt, the Health Secretary, controversially blamed this on GPs giving up responsibility for out-of-hours care in contract renegotiations in 2004 – an accusation they strongly denied.

Q | What are the alternatives to visiting A&E?

A | For non-urgent medical complaints the best place to go is still the GP surgery – and there are still 250 million GP attendances every year, compared to 21 million A&E attendances. In the past few years many walk-in centres and minor injury units have opened, which allow patients with minor conditions to see a doctor or nurse without an appointment. These kinds of wards are included in A&E statistics and account for the bulk of the recent increase in attendances. Increases at type 1 – or major A&E wards – has actually been relatively small.

Q | So why did so many miss their targets last year?

A | A&Es are expected to deal with 95 per cent of patients – either treating you and sending you home, or admitting you to hospital – in less than four hours. Last winter the number of people having to wait longer than that hit nine-year highs, ringing alarm bells about an A&E crisis. Nigel Edwards, a senior fellow at the King’s Fund health think-tank, believes that problems in A&E “are very often caused by our inability to discharge people from hospitals”. He points to a lack of capacity in the social care system, so when an elderly patient seeks emergency care at a hospital, it struggles to find somewhere safe for them to go after they have been treated.

That means more beds are taken up, and it takes longer to move people out of A&E into another ward, which in turn means it can take longer for doctors to see people who come to A&E, because resources are taken up by patients who, ideally, would already be in another ward.

Q | Why is the Government so concerned about A&E performance this winter?

A | Because admissions always go up in the winter months. The the Christmas bank holidays put pressure on the system, with many social care and other services shutting down, leaving hospitals to cope on their own. The great fear of any winter season – and one which played a part in the Government’s decision to inject an extra £250m of funding for A&Es this winter – is the risk of a major flu outbreak.