The UK's top nursing union has criticised new immigration rules that mean migrants must earn more than £35,000 (€48,600) to stay in the country, saying the regulations will cost the NHS millions of pounds.

The Royal College of Nursing (RCN) said not only will the new measures cost money but they will intensify a shortage in nurses and compromise patient safety.

The RCN said the new plans, which will affect individuals from outside the European Economic Area (EEA) and come into effect 2016, will force thousands of nurses to return to their own country.

The union said that 3,354 nurses in the UK would potentially be affected. It estimated that those nurses will have cost the NHS £20m in recruitment so far.

The total cost to the NHS is expected to increase if recruitment trends continue along current levels. By 2020 the number of nurses affected by the threshold would have reached 6,620, costing £39.7m.

Dr Peter Carter, chief executive and general secretary of the RCN, 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."

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. From 2016, non-EEA workers will need to earn at least £35,000 to settle in the UK for longer than six years.

"There are exemptions to this threshold for occupations where the UK has a shortage – but the independent Migration Advisory Committee recommended against adding nurses to the Shortage Occupation List after taking evidence from groups including the RCN."