Britain has been warned by the World Health Organisation to stop poaching foreign doctors and nurses from the world's poorest nations to shore up the NHS. The haemorrhage of health workers from the developing world is a key factor behind a global health workforce crisis which is costing millions of lives, the UN agency said.

At least 1.3bn people have no access even to the most basic health care, often because there is no health worker. The burden is greatest in countries hardest hit by poverty and disease.

The WHO yesterday named 57 countries that are short of 2.4 million doctors, nurses and midwives in its annual World Health Report. Essential life-saving measures such as childhood vaccination, pregnancy and maternity care and treatment for HIV, malaria and tuberculosis are threatened by the shortage.

"Not enough health workers are being trained or recruited where they are most needed and increasing numbers are joining a brain drain of qualified professionals who are migrating to better paid jobs in richer countries," Timothy Evans, WHO assistant director general, said.

In Canada, New Zealand, the United Kingdom and the United States, a quarter or more of all physicians had been imported from other countries. On average one in four doctors and one nurse in 20 trained in Africa is working in OECD countries.

Last year, a report by Save the Children and the charity Medact estimated that Britain had saved £65m in training costs for doctors and £38m for nurses it had taken from Ghana alone since 1999.

Estimates show that the US, which already has more than half of all the world's nurses, will need a further 800,000 by 2012 and cannot fill the gap from those trained at home. Instead it is expected to rely on its superior economic power to buy in nurses from abroad.

The 2006 World Health Report calls for urgent investment in training and the recruitment of extra health workers in the countries where they are most needed and providing incentives for them to stay.

It says health budgets will have to increase by at least $10 a person per year in the 57 countries with the most severe shortages to educate and pay the salaries of the extra four million health workers (including managers and public health specialists) needed.

But the report stops short of recommending measures to prevent health workers migrating, such as financial bonds to cover the cost of training or restrictions on exit visas. Manuel Dayrit, WHO director of human resources for health, said: "We know these don't work. And WHO supports the right of people to migrate and follow their aspirations."

More must be done to encourage doctors and nurses to stay, rather than preventing them from leaving, he said. In many countries, they were poorly paid and working in "objectionable" conditions.

Britain introduced an employment code for NHS trusts six years ago which bans international recruitment from countries with the worst shortages. But hospitals in the NHS and private sectors have found ways round it. Dr Dayrit said: "As Britain and the US look at their own health workforce needs they should be aware that some of their policies are having dire consequences elsewhere. They should look at the needs of the countries affected and create policies to help them."