Paul Boateng, the former cabinet minister, has become a director of a private military company after lobbying the South African government to water down proposed anti-mercenary legislation.

Boateng has joined the board of Aegis Defence Services, run by Tim Spicer, the former Guards officer at the centre of the arms-to-Sierra Leone scandal in 1998.

The disclosure last night prompted a row about the so-called “revolving door” of former ministers who take up lucrative private-sector posts after leaving office.

Boateng, who in the 1980s was regarded as a left-wing firebrand, stepped down as an MP in 2005 to become Britain’s high commissioner in South Africa.

He quit the post in May and last month became a director of Aegis, which has lucrative contracts in Iraq.

As high commissioner, Boateng made repeated representations to the Pretoria government on behalf of the Foreign Office about proposed legislation designed to prevent South Africans from joining foreign military forces.

He called for an amendment to ensure that South Africans who joined the British army were not forced to surrender their South African citizenship. However, the ANC-led government rejected

Britain’s proposed change, claiming it would have wrecked the bill by creating loopholes allowing South Africans not only to join foreign armies but also to become mercenaries with companies such as Aegis.

Spicer came to prominence in the late 1990s, when his previous military company Sandline International was revealed to have helped ship arms to coup plotters in Sierra Leone. The Falklands veteran is a former business associate of Simon Mann, the mercenary who was jailed for planning a putsch in Equatorial Guinea but released this week. Spicer was questioned by British officials but was not implicated in the incident.

It is not known how much Boateng is being paid by Aegis. However, another non-executive director, Nicholas Soames, the Conservative MP and grandson of Winston Churchill, declares in the register of members’ interests monthly payments of £4,916.80, equivalent to an annual salary of £59,000.

The South African mercenary legislation was drawn up in an effort to end the country’s reputation as a recruiting ground for “dogs of war” fighting in dirty conflicts in Africa and elsewhere. It is believed that South Africans are the second largest nationality after Americans among those working for private military companies in Iraq.

Boateng made several representations to Mosiuoa Lekota, then the defence minister, on behalf of the British army, which has 800 South African nationals in its ranks.

A spokesman for Lekota, who has since become president of a new political party, Cope (Congress of the People), said the minister had been concerned about possible “unintended consequences” of the British lobbying.

He added: “There was concern that the amendments being proposed by the British government would have not only allowed South African nationals to join the British army but also to join private military companies and therefore destroy the purpose of the legislation.”

A spokeswoman for Aegis said Boateng did not meet Spicer for the first time until September this year.

“Aegis had no contact with Paul Boateng when he was high commissioner of South Africa about the South African mercenary bill or any other matter. The high commissioner would have acted at all times on the instructions of Her Majesty’s government in making representations on this as on all matters.”

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