The former trade and industry secretary, who travelled to South Africa on Sunday, had hoped his visit would remain a secret and he would return to Britain without being recognised. But the news started to emerge in Pretoria last night.
It is believed that Mr Mandelson is on a fact-finding mission to discover whether he could take on a more formal role during the country's two-month election campaign.
The aim would be to help the ANC retain power and ensure a smooth transition from the regime of Nelson Mandela to Thabo Mbeki, his chosen successor as president.
The idea was suggested by Richard Caborn, Britain's minister of the regions, who has close links with the ANC and is a former anti-apartheid campaigner. It was encouraged by John Prescott, the Deputy Prime Minister and Mr Caborn's departmental boss.
They saw the move as a way of both aiding the ANC and helping Mr Mandelson's political rehabilitation following his resignation from the Cabinet in December after the revelation of his pounds 373,000 personal loan to buy a home in west London from Geoffrey Robinson, then paymaster-general.
Friends say that Mr Mandelson enjoys "celebrity status" in South Africa as a close ally of Tony Blair and leading architect of New Labour.
The prospect of him advising the ANC was discussed at Downing Street by officials from the South African Embassy in London in December. It was also raised when Mr Blair met Mr Mbeki during his three-day trip to South Africa last month.
This week's mission by Mr Mandelson was set up by Margaret McDonagh, the Labour Party's general secretary, and the trip was funded by British trade unions.
The ANC party machine is believed to be in need of an urgent overhaul before the election. Mr Mandelson could also give advice on the relationship between party and government. After masterminding Labour's 1997 general election campaign, Mr Mandelson became minister without portfolio, when he was responsible for the co-ordination and presentation of government policy.
Mr Blair is keen to forge closer links with the ANC, as part of his drive to give his "third way" philosophy an international dimension.
Mr Mandelson's links with Africa date back to 1973, when he spent a year as a voluntary worker in Tanzania.Reuse content