"Some 23,000 qualified academic professionals emigrate from Africa each year in search of better working conditions," the report said.

"The region (Africa) has lost an estimated 60,000 middle-and high-level managers between 1985 and 1990," it added.

The report was presented yesterday to an African preparatory conference for next year's World Summit on Sustainable Development, nicknamed the "Earth Summit" and due to be held in Johannesburg in September 2002.

Compiled by Chris Buckley of the Pollution Research Group at Natal University in South Africa, it cites World Bank estimates that 100,000 expatriates from the industrialised countries are employed in Africa at a cost of $4 billion per year.

"(This) is 35 percent of official development assistance directed to the continent," the report says.

Experts say employing expatriate workers, who are often more expensive than African professionals, makes the process of sustaining economic and environmental development even harder.

Buckley's report lists the brain drain as a "major cause of environmental degradation", and it is one of several topics due to be discussed at the conference in the Kenyan capital Nairobi.

Nearly 40 African ministers are attending the conference, which is intended to mould Africa's agenda for the World Summit.

The Johannesburg summit is a follow-up to the environmental summit in Rio de Janeiro, Brazil 10 years ago.