The investigation found that Iqbal Riza, Mr Annan’s chief of staff, ordered the shredding the day after the Security Council approved the inquiry last April, and the shredding continued until the week of December 7.

The documents covered the crucial period from 1997 to 1999, when the Swiss company that employed Mr Annan’s son, Kojo, as a consultant was awarded a lucrative UN border-inspection contract in Iraq.

Ten days before the shredding, Mr Riza had sent the heads of nine UN-related agencies a directive asking them to “take all necessary steps to collect, preserve and secure all files, records and documents . . . relating to the Oil-for-Food programme”.

Paul Volcker, who led the inquiry, said: “Whether that material contained any evidence that we did not otherwise get from UN files more generally is, of course, not known.”

The possible cover-up adds a new dimension to a scandal that has tarnished the reputation of the UN and dozens of politicians and prominent figures who allegedly received favours from Saddam Hussein. It comes at a time when the UN is also under fire for sexual abuse by its peacekeepers in the Democratic Republic of Congo, and sexual harassment and mismanagement by senior staff.

The three-member Volcker commission found that Kojo Annan “actively participated” in efforts to conceal the true nature of his relationship with the Swiss firm Cotecna Inspection SA. When news of this relationship broke in January 1999, Cotecna disguised payments to Kojo Annan by making them through other companies.

The report also says that Kojo Annan — who is no longer co-operating with the inquiry — teamed up with a partner to explore other business opportunities in Iraq under the Oil-for-Food programme and twice met Saddam’s ambassador.

“Significant questions remain about the actions of Kojo Annan during the fall of 1998, as well as about the integrity of Kojo Annan’s business and financial dealings with respect to the programme, and the committee’s investigation of these matters is continuing,” the inquiry said.

However, it found no evidence that he had influenced the award of the UN contract to Cotecna, and said the evidence was “not reasonably sufficient” to show that the UN chief knew about Cotecna’s bid.

The Secretary-General seized on those words last night to claim the inquiry had “cleared me of any wrongdoing”, and the White House said it still supported him.Asked by journalists if he would resign, Mr Annan replied: “Hell, no.”

But he expressed disappointment at the report’s findings about his son. “I love my son, and I have always expected the highest standards of integrity from him,” he said.

“I am deeply saddened by the evidence to the contrary that has emerged, and particulary by the fact that my son had failed to co-operate fully with the inquiry.”

Kofi Annan consistently denied that he knew Cotecna was bidding for the UN border inspection contract, although he had been aware of the firm’s earlier attempt to get another UN contract.

But the Volcker commission said that the Secretary-General “could have been alerted to the potential conflict (of interest)” in a number of ways, “including frequent conversations with his son”. It also said his internal investigation of the award of the contract had been inadequate.

The panel noted that Kofi Annan had met Elie Massey, Cotecna’s founder, shortly before the contract was awarded. It also revealed that Kofi Annan was a lifelong friend of Michael Wilson, the company’s vice-president for marketing operations in Africa, and a man whom called Mr Annan “uncle”.

Mr Wilson originally told investigators that he had alerted Kofi Annan to Cotecna’s possible interest in bidding for a UN contract in Iraq, but he called them back within 20 minutes to say he was mistaken. Mr Annan denied any such conversation ever took place.

The committee also found that Kojo Annan’s business partner, Pierre Mouselli, had lunch with the UN chief in South Africa in September 1998. “Mr Mouselli stated that he and Kojo Annan told the Secretary-General at this lunch about their work for Cotecna in Nigeria as well as their intention to form oil and inspections companies,” the report said. “Mr Mouselli did not remember the Secretary-General voicing any concerns.” But Kofi Annan told the inquiry that the meeting was “so brief I don’t think we had much substantive discussion”.

Mr Riza, who retired this year, said he had ordered documents to be shredded at his secretary’s request to save space and assumed there were copies. The inquiry said it did not find the explanation persuasive.

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