The United States Congress is demanding the right to hear from two investigators who quit the United Nations inquiry into the Oil-for-Food scandal because they felt that it was too soft on Kofi Annan.

Norm Coleman, a Republican from Minnesota who chairs the Senate Permanent Sub-committee on Investigations, has ordered his staff to subpoena Robert Parton and Miranda Duncan to testify.

The two Americans resigned from the UN inquiry last month after the panel, led by Paul Volcker, was said to have rejected two drafts that they had written that were highly critical of Mr Annan, the UN Secretary-General.

Mr Volcker, a former head of the US Federal Reserve, has telephoned the chairmen of three congressional committees investigating the scandal in an effort to prevent them issuing subpoenas. The UN insists that the investigators are protected by diplomatic immunity.

Senator Coleman, who has called on Mr Annan to resign for mismanagement, vowed to force a showdown by issuing the subpoenas anyway. “I spoke with Mr Volcker and expressed my grave and growing concerns about the credibility and independence of the investigation into the criminal misconduct that occurred in the UN Oil-for-Food programme,” he said.

Questions have been raised about Mr Volcker’s impartiality by the resignation of the two investigators and by his ties to a company once run by Maurice Strong, a Canadian tycoon and diplomat under investigation by the Volcker panel.

The inquiry said in March that it had found “no evidence” that Mr Annan had rigged the bidding to award a UN border-inspection contract in Iraq to a Swiss company employing his son, Kojo.

Mr Annan has exonerated Iqbal Riza, his former chief of staff, whom the Volcker panel found had shredded three years of documents, starting just a day after the UN Security Council authorised the Volcker inquiry. The UN chief called the shredding “careless.”

The UN did, however, unearth an apparent violation of staff rules by Mr Strong, who stepped aside temporarily as the UN envoy to North Korea last month while the Volcker commission investigates an investment he received from Tongsun Park, a North Korean-born businessman charged in the US with acting as a secret lobbyist for Saddam Hussein’s Iraq. The UN said that Mr Strong had hired Christina Mayo, his stepdaughter, to work in his UN office in 2003. Ms Mayo resigned last week.

In the 1960s Mr Strong, 76, ran Power Corporation of Canada, which later hired Mr Volcker as a paid consultant.

The UN is refusing to reveal whether Mr Strong played any role in recommending Mr Volcker for his post as head of the Oil-for-Food inquiry.

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