The past month, the spotlight has been on James Ibori, the governor of Nigeria's Delta state from 1999 to 2007, who pleaded guilty in a London court to 10 counts relating to conspiracy to launder funds from the state he governed.

Ibori was accused of siphoning off an estimated $250m and laundering it in London through a number of offshore companies and financial intermediaries to fund his extravagant lifestyle of lavish mansions, expensive cars and private jets. This mode of illicit capital flight is by no means restricted to one rogue Nigerian governor or even African leaders at large, nor is it the most important means by which capital leaves the continent (and developing countries generally) illicitly.

True, $250m from one source is substantial. But this pales into insignificance compared with the estimated $100bn that left Nigeria illicitly between 1970 and 2008, according to Global Financial Integrity (GFI). The bulk of this haemorrhage, contrary to popular belief, is not through the laundering of corrupt money but through commercial activities, and particularly through multinational corporations.

According to GFI's conservative estimates, more than $1.8 trillion left African shores illicitly between 1970 and 2008. Of this, only 3% is attributable to bribery and theft by government officials, 30%-35% results from the laundering of criminally acquired wealth (drugs, illegal arms sales, human trafficking, etc), and the bulk – 65%-70% – is from commercial activities, especially through trade mis-pricing of goods.

Over the last 10 years, the average annual outflows of this sort exceeded $50bn. This compares with annual aid inflows of less than $30bn. The outflows are largely to avoid or evade tax and to conceal wealth.

This week's proposed change by the chancellor, George Osborne, on how foreign subsidiaries of multinationals based in the UK are taxed, will give even less incentive to keep money in poorer countries. Reform of these controlled foreign company rules in the upcoming budget would strengthen the financial case for shifting money to tax havens by making profits made by multinationals abroad and retained in offshore jurisdictions free from UK tax. This could cost developing countries £4bn a year in lost tax revenue, according to ActionAid estimates.

These outflows undermine the rule of law, stifle trade and worsen macroeconomic conditions. They are facilitated by around 60 tax havens and secrecy jurisdictions that enable the creating and operating of millions of disguised corporations, shell companies, anonymous trust accounts and fake charitable foundations. They allow the likes of Ibori and many multinational corporations to cripple Africa financially and politically.

Given that about 50% of global trade passes through tax havens, these jurisdictions facilitate trade mis-pricing by making it difficult for documentation to be traced. Transnational companies have the ability to set up multiple trusts and shell companies in these jurisdictions. This is significant because about 60% of global trade takes place between and within multinational companies. Secrecy also attracts criminal activity, and the laundering of corrupt money through concealment of the natural beneficiaries behind shell companies and trusts.

Africa is experiencing economic growth, and for the increasing wealth to be channelled to public services, development and the achievement of the millennium development goals by 2015, it is urgent the problem of tax havens as a conduit for illicit outflows is addressed. The high-level panel set up by the African Union, the African Development Bank and the UN Economic Commission for Africa, and chaired by former South African president Thabo Mbeki, is a significant step forward – and testifies to the importance of this issue for Africa's development. The ball is now in the court of the rich countries.