The NHS is "sleepwalking" into a nursing crisis with thousands of frontline posts lost and training positions axed, the Government is warned today.

The Royal College of Nursing (RCN) said that despite the Coalition's promise to protect frontline staff from cuts the NHS workforce has fallen by almost 21,000 since the Coalition Government came to power. This includes a loss of more than 6,000 qualified nursing posts – from a total of 312,000 nursing posts in the NHS.

The RCN's report also warns that parts of the health service face the prospect of nursing shortages within three years as thousands of training posts are slashed, meaning trusts will have to recruit from overseas.

Patient safety will be seriously undermined by falling numbers of nurses, with the RCN's chief executive warning that standards of care "are going to get a lot worse".

The nursing union has been tracking job cuts since the Coalition came to power in May 2010. It has found that about 1,000 posts are being earmarked as "at risk" by NHS trusts every month as they try to find savings of £20bn during this parliament.

As well as job losses, the number of new nurses being trained has fallen sharply, by 14 per cent in just two years. In London, training places for adult nurses have fallen by 21 per cent, which will lead to substantial shortages by 2015 – highlighting failures in long-term workforce planning, warns the RCN. District nursing is heading towards crisis, as numbers of nurses have plummeted by a third since 2001 to 8,000.

The RCN questions how the NHS can re-focus care from hospitals to the community – essential for improving patient outcomes and saving money – if the cull of district nurses continues.

Nurse leaders warn today that the Government will soon be stranded in a "perfect storm" of an ageing population with increasing numbers of long-term conditions without enough nurses safely to care for patients.

Peter Carter, the RCN's chief executive, said: "London is facing a workforce crisis within three years. The remedy will be to go overseas to countries like the Philippines to raid their workforce again, and an over-reliance on agency and temporary staff – in order to bail out the Government's poor workforce planning." He added: "The standards of care are under huge strain across England and if this trajectory continues unchecked then things are going to get a lot worse. There is no rogue information in our data. This is not the worst-case scenario: it is the declared scenario from trusts."

The pledge to protect frontline staff was a key Coalition promise even as it announced the need to save £20bn to cope with increasing healthcare demands as budgets flatline after years of record investment.

But official figures reveal that there were almost 6,150 fewer full-time equivalent qualified nurses in July this year compared with May 2010 despite Coalition promises to protect frontline staff. In total, there are 20,790 fewer NHS staff, but the number of doctors has increased by 7,000, according to the NHS Information Centre.

The long-awaited report into the Mid Staffordshire hospital scandal is expected to recommend minimum nursing levels to improve patient safety in hospitals. On average there is one qualified nurse to every four paediatric patients, but only one for every nine elderly patients.

Yet there is compelling evidence from King's College London that patient outcomes improve when science is applied to nurse-patient ratios – in short, making sure there are enough nurses safely to care for patients in different settings.

The Government has repeatedly dismissed the RCN's figures as scaremongering, but does not monitor proposed cuts by NHS trusts centrally. Furthermore, the growing number of NHS contracts being awarded to private companies such as Circle, Virgin Health and Care UK will soon make it even more difficult to track job losses. Private companies do not submit such data to the NHS Information Centre, nor will workforce plans be available for public scrutiny.

The Health minister Dr Daniel Poulter said: "NHS performance is strong: waiting times and infection rates are at record low levels. To say that the NHS is in 'crisis' is scaremongering and doesn't reflect reality.

"The health service is changing – the workforce is changing to reflect this, but changes must be decided at a local level, based on evidence that they will improve patient care."