The decision, which took the country by surprise, follows the presentation of a military intelligence report to Mr Mandela endorsed by Georg Meiring, chief of the South African National Defence Force, which apparently suggested that a coup had recently been plotted.
Last night it seemed that the government was taking the possibility of a malicious motive being behind the report as seriously as the allegations made in it. The gravity of the situation is reflected in the heavy weight composition of the inquiry team. It will be headed by the Chief Justice, Judge Ismail Mahomed. The two other members are Judge Richard Goldstone, a member of the Constitutional Court and a UN War Crimes prosecutor, and Justice Pius Langa, deputy president of the Constitutional Court.
That the inquiry begins today in camera at a secret location is also a sign of the government's concern. The team has been ordered to report its finding as soon as possible.
The inquiry follows weeks of press speculation about the bizarre case of Robert McBride, a senior foreign affairs official and former ANC guerrilla, arrested in Mozambique with a stack of AK47s and $11000 in his pocket.
Mr McBride is in jail in Maputo awaiting trial for arms smuggling. Right- wing newspapers have claimed that he was working for senior members of the police and military forces, planning a coup.
But Mr McBride's wife, Paula, claims he was involved in a deep throat operation to flush out arms dealers. The security services have denied that Mr McBride was working for them, and the government has distanced itself from claims that he was an undercover agent.
To add further complications, Mr McBride, infamous for his planting of a bomb in a Durban bar in 1986, is believed to have right wing enemies prominent in police and armed forces.
That the Mandela government may be more interested in the motives behind the report, than the allegations in it, was evident yesterday in a statement from Mr Mandela's office, which said the inquiry would look into the "process relating to the compilation, verification and subsequent treatment of the report". A presidential aide said the inquiry would try to establish why a coup plot was spoken of - and leaked to the press - when no evidence existed to support its existence.
The government has frequently complained that senior members of the police and army have not bought into the new South Africa and are working to undermine it.