Plans to deport more than 3,000 immigrant nurses will waste millions of pounds, cause "utter chaos", and risk compromising patients safety, the Royal College of Nursing (RCN) has claimed.
New measures unveiled by the Government on Monday will boot out migrants earning under £35,000 a year after six years of residence. The rules come into affect next year.
The RCN has calculated that up to 3,365 nurses currently employed in the UK will potentially be affected and that £20.19 million in NHS money will have been wasted in recruiting them.
It added that if international recruitment stays the same as it is now, by 2020 the number of nurses affected by the threshold will be 6,620, employed at a cost of £39.7 million.
According to the union, a newly qualified nurse earns around £21,000 a year and a "very experienced" nurse takes home about £31,000-£35,000.
The Home Office said it wanted to reduced the demand for migrant labour and that hospitals had had since 2011 to prepare for the rule change.
But the RCN has claimed the scheme will only intensify a severe shortage of nurses and leave hospitals with nothing to show for the millions of pounds spent on recruiting them.
Dr Peter Carter, chief executive and general secretary of the union, said: "The immigration rules for health care 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."
He said the only way for the UK to "regain control" over its health service was to train more nurses: "37,000 potential nursing students were turned away last year so there are people out there who want to embark on a nursing career."
A Home Office spokesman said: "As the Prime Minister has made clear, the Government wants to reduce the demand for migrant labour.
"We changed the settlement rules in 2011 to break the link between coming to work in the UK and staying here permanently."
The spokesman added that there were "exemptions to this threshold for occupations where the UK had a shortage".
However, the spokesman said: "The independent Migration Advisory Committee recommended against adding nurses to the Shortage Occupation List after taking evidence from groups including the Royal College of Nursing."
David Cameron, referring to the Migration Advisory Committee, on Monday defended their decision.
"They haven't put nursing on that shortage occupation list and I think we should listen to their advice above all," he said.