Outgoing Prime Minister Tony Blair is to intervene this week to try to prevent 700 South Africans - the equivalent of more than one battalion - being forced to quit the British Army because of a proposed anti-mercenary law.

Mr Blair is to raise the issue in talks with Thabo Mbeki, the South African president, during his farewell tour of Africa. The South African parliament passed the Prohibition of Mercenary Activities Bill late last year, although it still has to be signed off by Mr Mbeki.

The draft legislation is aimed at curbing an estimated 20,000 South Africans hiring themselves out as soldiers of fortune in various Third World conflicts, or volunteering for foreign armies. It followed a spate of bad publicity involving bungled mercenary coups such as the one in Equatorial Guinea two years ago in which Mark Thatcher, son of the former prime minister, was alleged to have played a role in bankrolling the plotters.

The new law would also make it illegal for thousands of South Africans working for private security firms in Iraq as bodyguards, convoy escorts and intelligence advisers to ply their trade.

Meanwhile, South Africa again rejected calls for tough action against Zimbabwe ahead of Mr Blair's visit. South African Foreign Minister Nkosazana Dlamini-Zuma said yesterday in a speech to parliament a hard line on Zimbabwe would only backfire.