The swine flu pandemic could kill millions and cause anarchy in the world's poorest nations unless £900m can be raised from rich countries to pay for vaccines and antiviral medicines, says a UN report leaked to the Observer.

The disclosure will provoke concerns that health officials will not be able to stem the growth of the worldwide H1N1 pandemic in developing countries. If the virus takes hold in the poorest nations, millions could die and the economies of fragile countries could be destroyed.

Health ministers around the globe were sent the warning on Thursday in a report on the costs of averting a humanitarian disaster in the next few months. It comes as officials inside the World Health Organisation, the UN's public health body, said they feared they would not be able to raise half that amount because of the global downturn.

Gregory Hartl of WHO said the report required an urgent response from rich nations. "There needs to be recognition that the whole world is affected by this pandemic and the chain is only as strong as its weakest link. We have seen how H1N1 has taken hold in richer nations and in the southern hemisphere. We have been given fair warning and must act soon," he said.

The report was drawn up by UN officials over the last two months. It was commissioned in July after Ban ki-moon, the UN's secretary general, expressed concern that the H1NI virus could have a severe impact on the world's poorest countries.

It paints a disastrous picture for the world's most vulnerable people unless there is immediate action. "There is a window in which it will be possible to help poor countries get as ready as they can for H1N1 and that window is closing rapidly," it says.

"Countries where health services are overburdened by diseases, such as HIV/Aids, tuberculosis and malaria, will have great difficulty managing the surge of cases. And if the electricity and water sectors are not able to maintain services, this will have serious implications for the ability of the health sector to function.

"If suppliers of fuel, food, telecommunications, finance or transport services have not developed plans as to how they would continue to deliver their services, the consequences could be significantly intensified," it adds.

The 47-page report provides a detailed breakdown of the basic needs of 75 vulnerable countries with the weakest capacity to withstand an escalation of the virus. Six countries from Latin America, including Cuba and Bolivia, 21 countries from Asia and the Pacific such as North Korea and Bangladesh, and 40 countries from Africa such as Congo and Eritrea are included in the survey.

UN officials say in the report that £700m should be spent on antiviral drugs and vaccines to protect health care workers and other essential personnel as well as cover those suffering from severe illness. They have identified 85 countries that do not have the ability to access vaccines from any other source and intend to cover 5-10% of each population.

A further £147m should be put aside to organise vaccine campaigns, improve communications, monitor levels of illness and improve laboratory capacity in 61 countries, the report claims. The remainder should be used to pay for the WHO and other UN-related organisations to help in these countries as well as an emergency fund for additional antiviral medicines, it argues.

The UN's efforts were boosted last week when nine countries, including Britain and the US, pledged to give the equivalent of a 10% share of their swine flu vaccine supply to help fight the deadly virus's global spread. In Britain, Douglas Alexander, the development secretary, pledged to give £23m.

Some officials within WHO believe, however, that this will not be enough. One said that richer countries were reluctant to pay out all of the money that was needed. "The downturn means that governments countries are reluctant to give," he said.

Another said: "The money is a trickle, not a flood. It is going to be a struggle. If we are not careful, the virus could destroy a burgeoning economy or democracy."

The UN's request for the money comes as the virus begins to establish itself in some of the world's most vulnerable countries. On Wednesday, health officials told one website that the African continent had recorded 8,187 confirmed cases of swine flu and 41 deaths.

Swine flu was declared a pandemic in June and has since been identified in 180 countries. Pandemic experts believe that the western world, including Britain, is facing a second wave of the virus.