LONDON (Reuters) - The world's wealthiest countries agreed on Saturday to write off more than $40 billion of African debts.

The deal struck by finance ministers from the Group of Eight industrialized nations is part of a British-led campaign to rid sub-Saharan Africa of poverty and diseases such as malaria and AIDS that kill millions every year.

British Finance Minister Gordon Brown said the deal would provide 100 percent write-offs "immediately" for 18 countries and that more countries would qualify for relief later.

"This is the most comprehensive statement the finance ministers have evermade on debt and poverty," he told a news conference.

"This is not a time for timidity, this is a time for boldness."

The accord covered debts to international lending agencies such as the World Bank, African Development Bank and International Monetary Fund.

Britain, chairing the G8 club this year, was determined to get an accord on debt at the talks in London but faces a far harder time drumming up support for a doubling of aid to Africa ahead of a meeting of G8 leaders inScotland next month.

Pop star Bob Geldof and others are urging a million people to turn up in Scotland to demand a deal on aid for Africa.

Brown had sought backing for an International Finance Facility (IFF) thatwould double aid to the poorest countries to $100 billion by issuing bonds using rich nations' development budgets as collateral.

But Washington has opposed the plan so Brown is likely to launch a pilot IFF project instead that would provide funds for vaccination programs in Africa without U.S. or Japanese support.

Turning to their own economic problems, the ministers issued a communique saying growth was likely to be a little less strong this year than last and renewing declarations that high world oil prices and other imbalancesposed risks.