Fraudulent trade invoicing in five African countries cheated taxpayers out of a combined $14.4 billion (8.5 billion pounds) in revenue in the 10 years to 2011, and in Uganda's case losses amounted to an eighth of annual government revenue, research showed on Sunday.

The tax authorities in the five countries studied by Global Financial Integrity (GFI) - Ghana, Kenya, Mozambique, Tanzania and Uganda - lacked the trade, tax and deals data to curb the illicit flows, it said in a research report.

Over- and under-invoicing in the five countries facilitated the illegal inflows or outflows of more than $60 billion during that decade, GFI said.

Kenya lost an estimated $1 billion each year through export under-invoicing, where sellers deflate the true value of their exports so they can channel the difference to a foreign account.

Tanzania, on the other hand, lost a similar amount to export over-invoicing, which over-values shipments so parties can collect export credits.

Uganda had $813 million in import over-invoicing, which can lead to lower corporate taxes as companies puff up the cost of imports to hide capital outflows.

GFI said its study was "extremely conservative" as it left out incorrect invoicing for services, bulk cash deals or hawala transactions, a form of money transfer used in the Muslim world.

"Trade misinvoicing is perhaps the most serious economic issue plaguing these countries," GFI president Raymond Baker said in a statement.

GFI is a non-profit research and advocacy organisation based in Washington that aims to curtail illicit financial flows out of developing countries.

Ghana had more than $14 billion in mis-stated invoices over the entire 10-year period, equivalent to 6.6 percent of its gross domestic product, while Mozambique's $5.3 billion was equal to 9 percent of national output.

Another report by the African Development Bank in 2013 showed net outflows from Africa's resources sector totalled up to $1.4 trillion over the three decades up to 2009, far exceeding inflows.