Johannesburg - The alleged corruption in South Africa's R43 billion armaments purchase should initially be subjected to a preliminary investigation headed by a retired judge or chief magistrate and assisted by independent forensic auditors, United Democratic Movement leader Bantu Holomisa said in an open letter to President Thabo Mbeki on Tuesday.

In the letter Holomisa said: "In the event that the Heath special investigating unit is excluded (seeing the Minister of Justice has so recommended) the president is under a compelling obligation to establish a judicial commission... that will be endowed with the powers, expertise and commitment that have characterised the Heath SIU in order to ensure that a credible investigation is carried out."

But a preliminary investigation could establish if it was necessary to set up a judicial commission. This was advisable given the high cost of a judicial probe, said Holomisa.

"Should the preliminary investigation establish prima facie evidence that there are irregularities in the transactions, then there is a weighty obligation on the part of the president, which you cannot ignore, to authorise a credible judicial commission to scrupulously probe the deal."

As the executive was now in the spotlight over allegations of irregularities in procurement procedures in the arms deal, the preliminary investigation team should include Parliament's standing committee on public accounts, which had recommended the probe.

"It is important, Mr President, that the public is not persuaded to believe that the Heath SIU is welcome as long as it does not investigate the current government," Holomisa said in the letter.

"Should you accede to the route of a preliminary investigation, the standing committee on public accounts (and not the executive which is the subject of the probe) should be empowered to interview a short list of people with the required expertise to carry out the investigation.

"I have no doubt that this gesture would calm the political turbulence sparked off by these revelations and create a favourable climate for an objective investigation."

Holomisa said there were "strong perceptions and suspicions of a cover-up as a result of the ruling party's utterances and the unmistakable hostility of some government ministers towards the Heath special investigating unit".

He warned that the government's conduct in dealing with the situation would be closely watched by the investment community and would affect their view of South Africa and the region as an investment destination.

With acknowledgement to Sapa and News24.co.za.