Proposals aimed at stopping private agencies recruiting nurses, and doctors, from developing countries have been published by the government.
The RCN warns countries are losing essential staff
There is an existing code in the England - but it only covers NHS recruitment of permanent staff.
Ministers say the extra proposals would also prevent the NHS or private healthcare providers recruiting temporary staff from these countries.
But nurses leaders say the government has not gone far enough.
The Royal College of Nursing (RCN) says the code should be mandatory, rather than voluntary.
Nursing staff from Africa are coming to work in the UK because pay is so much higher here than at home.
More than 1,300 nurses arrived in Britain last year from South Africa, with another 500 coming from Nigeria.
The Department of Health is hoping to encourage private companies to sign up by offering them access to NHS international recruitment programmes.
It hopes this will stop the process where the staff recruited from private companies then end up working in the NHS - so-called "back door' recruitment from countries which cannot afford to lose nurses and doctors to the UK.
It also wants to extend the code to around 200 more agencies that work with the NHS on domestic recruitment - currently only agencies supplying overseas healthcare staff have to comply with the code.
The code currently applies to 178 agencies and restricts recruitment from over 150 developing countries, including South Africa, other African nations and Asian countries.
Launching the proposals, Health Minister John Hutton said: "The UK already leads the way in terms of ethical recruitment practices.
"We were the first country to produce guidance on international recruitment based on ethical principles, the first to develop a robust code of practice for employers and the only country to publish an approved list of agencies.
"We are determined not to destabilise the healthcare system of developing countries. The NHS is expanding, but we're not going to do that at the expense of other countries."
He added: "We are working with the private sector to ensure it follows the NHS' example.
"These proposals will strengthen the code even further, bring the private sector into line with the NHS and will ensure that, as with Independent Treatment Centres, NHS contracts go to those signed up to the code.
"We're also proposing closing the loophole that allows trusts to recruit temporary staff from developing countries and extending the code to cover even more recruitment agencies.
"The revised code will ensure the NHS recruits staff ethically and fairly."
The Department has said it aimed to publish a revised code later this year.
But Beverly Malone, General Secretary of the RCN, said: "Whilst the changes proposed today go some way towards tightening up recruitment procedures in the NHS they do not go anywhere near far enough.
"If the government is serious about halting the depletion of nurses from developing nations, they must make sure that all employers in the UK are bound by the same rules.
"We recognise that some employers outside the NHS have voluntarily chosen to sign up to the Department of Health Code, but many more will not unless they are required to do so."
Shadow Health Minister John Baron added: "If the government is serious about stopping the NHS recruiting from developing countries then it could ban the NHS from using agencies that have not signed up to its code of practice.
"Their failure to take this simple measure suggests they are happy to turn a blind eye in order to help solve the recruitment retention problem at home."