Our government must do all it can to help clear up this murky African affair

The arrest of Sir Mark Thatcher by the South African authorities for allegedly helping to plot the overthrow of the government of Equatorial Guinea reads like a twist in a rather implausible thriller. All the ingredients of the airport bookshop novel are there: exotic locations; soldiers of fortune, mysterious puppet-masters, political intrigue and, naturally, huge sums of cash. But the fabulous nature of this affair should not distract from the seriousness of the charges against Sir Mark and the men already on trial for plotting a coup against the government of this small, oil-rich country.

The arrest of Sir Mark Thatcher by the South African authorities for allegedly helping to plot the overthrow of the government of Equatorial Guinea reads like a twist in a rather implausible thriller. All the ingredients of the airport bookshop novel are there: exotic locations; soldiers of fortune, mysterious puppet-masters, political intrigue and, naturally, huge sums of cash. But the fabulous nature of this affair should not distract from the seriousness of the charges against Sir Mark and the men already on trial for plotting a coup against the government of this small, oil-rich country.

It is difficult, of course, to summon up much sympathy for the present rulers of Equatorial Guinea who would have been deposed by such a coup. President Teodoro Obiang Nguema has been responsible for brutal human rights violations since he came to power in 1979. His claim to democratic legitimacy is based on intimidation of opponents and rigged elections. Only the president's cronies have benefited from the oil revenues that have come in since vast gas and oil deposits were discovered in the Nineties. The 500,000 inhabitants of Equatorial Guinea would, in all likelihood, be better off without him.

Nevertheless, it is wrong to suggest that it is legitimate for a collection of freelance soldiers to take matters into their own hands. The concept of hired mercenaries deposing governments is anathema to the international rule of law. Private defence companies have no business instigating or facilitating "regime change", even in countries ruled by tyrants as vile as President Obiang. And that nation's enormous oil reserves suggest that, in the event of a coup, the motives of the plotters would probably not be humanitarian in nature.

Private military companies that tout their services to governments or companies willing to pay them are increasing in number. The US government has made great use of them in Iraq. Britain's own Foreign Secretary endorsed the "outsourcing" of certain military tasks to the private sector in 2002. If the charges against the private soldiers in this affair are borne out, the question will be raised whether Western governments have ignored, even nurtured, the growth of an unaccountable khaki-clad monster.

In this context, it is commendable that the government of South Africa has chosen to crack down on the dubious freelance soldiers who operate within its borders. Turning a blind eye to such operators, who are immensely skilful in enriching themselves but not the country in which they take up residence, would be a strange way of trying to stabilise the fractured African continent. The British government ought to follow South Africa's lead and do all it can to assist in these investigations.

The international complexion of the group already on trial in Equatorial Guinea and Zimbabwe - white South Africans, Angolans and Armenians are among those involved - should not obscure the evident British connection. The alleged leader of the plot is an old Etonian former SAS officer, Simon Mann. Ely Calil, another figure named by the prosecutor in Equatorial Guinea as involved in the alleged plot, lives in London. Other names that have been thrown into the equation, albeit tangentially and amid strong denials, include the disgraced peer Jeffrey Archer and the former adviser to Margaret Thatcher, David Hart. It is in the interests of any government whose citizens are caught up in this affair to show that there was no collusion, or tacit encouragement, at any level with a plot of this kind.

This is a murky business. There have been allegations that the police in Equatorial Guinea extracted information from detainees through torture, which should raise doubts about its reliability. The fact that some of the alleged plotters are being tried in Zimbabwe, a country run by someone as contemptuous of due process as Robert Mugabe, is serious cause for concern. No one has yet been found guilty, and the evidence has yet to be presented. But one thing is already certain. Mercenaries are no blessing to Africa, but a curse.