The world's poorest people are being denied access to drugs because pharmaceutical companies are focusing their resources on diseases suffered by wealthy, middle-aged Americans, such as obesity and heart disease, a leading expert will say tomorrow.

Dr David Rhodes, the Health Protection Agency's (HPA) head of business development, will claim that spiralling costs are driving firms to invest primarily in drugs that tackle diseases of 'older Americans'.

As a result, the international market has been flooded with medicines to treat 'American diseases' such as high blood pressure, obesity, heart disease and cancer, while drugs to tackle tuberculosis, malaria and water-borne diseases prevalent in the poorest countries have been neglected.

Presenting his research at the HPA's annual conference tomorrow, Rhodes will show that more and more pharmaceutical companies are moving their headquarters to the US in search of profits. Once there, they pump money into treatments that help the local population to live longer.

'Drugs and vaccines are becoming phenomenally expensive to develop,' said Rhodes. 'Companies have to recoup their investments by selling the drugs and vaccines. To be economic, they need a large population and the price has to be high. That increasingly means that drugs are developed for older Americans, who are getting healthier and living longer.'

Costs are soaring, added Rhodes, because of extensive safety and efficacy testing and the fact that many drugs that show 'early promise' never make it through the checks.

As such, companies looking to be 'economic' shift resources to meet the needs - and benefit from the profits - of the biggest spenders. 'The US tend to get the first bite of the cherry,' admitted Rhodes.

He said the trend had led to a 'vicious' circle in the poorest countries of 'low economic growth leading to poor healthcare systems, creating a higher burden of disease which in turn affects the ability of the population to develop economically'. But while sub-Saharan Africa is heavily affected, China and India's strong investment in their pharmaceutical industry has seen health improvements and economic bonuses that will in turn attract investment back.

Nevertheless, with many private companies turning their back on the developing world, Rhodes said research was heavily dependent on philanthropic funding and government backing.

He welcomed the International Finance Facility for Immunisation - the funding arm of the Global Alliance for Vaccines and Immunisation - that was launched on Friday. The group has pledged to raise $4 billion (£2.2bn) for an immunisation programme in the developing world.