New stringent immigration rules will fuel a critical shortage of nurses in Britain, “cause chaos” in hospitals and cost the NHS millions, according to alarming projections carried out by the Royal College of Nursing (RCN).

Up to 3,365 nurses currently working in Britain are likely to be affected by a policy which says migrants from outside Europe must earn £35,000 or more if they are to be allowed to stay on after six years working here. Nurses rarely receive such salaries – particularly not within six years of starting work in the NHS.

The loss of these skilled clinicians will compromise patient safety, the nurses’ union says. As nurses with rejected visas return to their home countries it would also mean the NHS had wasted more than £20m on the recruitment of staff who can no longer stay on. The effects of the new rules will start to be felt in 2017.

Cuts to nurse-training places mean trusts are relying more on overseas recruitment and temporary agency staff to plug the gaps. Agency spending on doctors and nurses has soared from £1.8bn to £3.3bn in the past three years. A cap on outsourced staff combined with the new immigration restrictions will mean hospitals are doubly limited in their ability to keep up safe staffing levels, the RCN says.

Dr Peter Carter, the chief executive and general secretary of the RCN, said: “The immigration rules for healthcare workers will cause chaos for the NHS and other care services. At a time when demand is increasing, the UK is perversely making it harder to employ staff from overseas.

“The NHS has spent millions hiring nurses from overseas in order to provide safe staffing levels. These rules will mean that money has just been thrown down the drain. The UK will be sending away nurses who have contributed to the health service for six years. Losing their skills and knowledge and then having to start the cycle again and recruit to replace them is completely illogical.”

If no changes are made to current policies, and a shortage of home-grown nurses continues, by 2020 the number of nurses affected by the threshold will be 6,620, employed at a cost of £39.7m.

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Evidence of an expected loss of experienced migrant nurses follows research from Oxford University showing nurses are likely to be among the hardest hit by tighter rules governing who comes to work in Britain in the first place.

Since 2011, there has been an annual cap of 20,700 on all standard visas granted to non-EU workers. When the monthly cap is exceeded – which it was within 11 days this month – cases are decided based on those with the most points. More points are awarded for having a higher salary or being in a job on the Shortage Occupation List. Nursing is not currently on this list and since it is low-paid, applicants for these jobs are more likely to be rejected.

Madeleine Sumption, the director of the Migration Observatory at Oxford University, said: “We found that with the increased demand for these visas the salary threshold went up sharply and some occupations were hit harder than others. The professions that get shut out are the ones that pay less, like nurses. They are likely to be hit harder because they’re a skilled group that’s paid less.”

Yvette Cooper, the shadow Home Secretary, said: “The Government is delivering the worst of all worlds. They have not provided enough training places for nurses here in Britain, and now they want to send away the fully trained nurses who have been working here for many years even though the local NHS want them to stay, and there are no local staff to fill the gap.

“This Government’s decision to cut training places has meant more and more NHS trusts have had to recruit from abroad instead. The RCN has been warning for many years about the problem, but the Government has failed to act.

“The Home Office and the health department need to work together urgently to sort this out.”

The RCN believes a longer-term solution is to train more nurses. Dr Carter said: “The only way for the UK to regain control over its own health-service workforce is by training more nurses. Some 37,000 potential nursing students were turned away last year.

“There are clear signs of a global nursing shortage, meaning an ongoing reliance on overseas recruitment is not just unreliable but unsustainable.”